Airbag

1. An airbag configured to expand between two seats that are arranged in a vehicle in a direction of vehicle width,

wherein the airbag is configured to be installed in a case located behind seating positions of the two seats, and when a gas is provided to the airbag, the airbag is inflated and expanded towards a front side of the vehicle so that an upper end and a lower end of the airbag are positioned on a front side relative to the case,

wherein in an expanded state, an outline of a vertical cross section of the airbag taken along the vehicle width direction is shaped to have its widest portions located at a vicinity of a center in a height direction, and

wherein in the expanded state, the upper end of the airbag has a substantially flat plateau so as to be securely abutted to an interior ceiling of the vehicle, respective outermost parts of the airbag are arranged between respective inner ends of headrests of the two seats when viewed in a front view from the vehicle front side, and the width of the airbag in the vehicle width direction is gradually decreased from the vicinity of the center in the height direction toward a top so that a space is secured between the airbag in the expanded state and a head and a neck of an occupant.

2. The airbag according to claim 1 , wherein in the expansion thereof, the airbag forms a three-dimensional shape composed of a front face, a back face, an upper face, a lower face, and left and right side faces protruding in a convex manner to both sides in the vehicle width direction.

3. The airbag according to claim 1 , wherein in the expanded state, the outline of the vertical cross section along the vehicle width direction has a polygonal shape.

4. The airbag according to claim 3 , wherein the polygonal shape has a polygonal number of an even number of 4 or more.

5. The airbag according to any of claims 1 to 2 , wherein the airbag is planarly configured by two left and right flat fabric cloths.

6. An airbag configured to expand between two seats that are arranged in a vehicle in a direction of vehicle width,

wherein the airbag is configured to be installed in a case located behind seating positions of the two seats, and when a gas is provided to the airbag, the airbag is inflated and expanded from the case towards a front side of the vehicle so that an upper end and a lower end of the airbag are positioned on a front side relative to the case,

wherein the airbag has an outline shape in a front view as seen from the front side of the vehicle in an expanded state that is composed of an airbag upper part that forms a part above a height position corresponding to a shoulder of a seat back of each of the two seats and an airbag lower part that forms a part below the height position corresponding to the shoulders of the seat backs and that protects a shoulder and an arm of an occupant, the airbag upper part has outermost parts expanded up to a vicinity of an inner end of a headrest of each of the seat backs in the vehicle width direction in the front view, the airbag lower part has the outermost parts expanded up to a vicinity of respective inner ends of the left and right seat backs in the vehicle width direction in the front view, and the air bag upper part and the air bag lower part are configured and shaped to be connected to each other in a stepwise manner so as to retain the head, the neck and a chest of the occupant at substantially the same timing when deployed,

wherein in the expanded state, the upper end of the airbag has a substantially flat plateau so as to be securely abutted to an interior ceiling of the vehicle, and

wherein in the expanded state, a cross-section of the airbag lower part that is taken along a substantially horizontal plane located below and adjacent to the height position corresponding to the shoulder of the seat back has a recess to accommodate the shoulder and arm of the occupant on each side at a position on a front side of the seat back.

7. The airbag according to claim 6 , wherein the airbag is configured, in the expanded state thereof, so that an upper face of the airbag upper part is abutted to the interior ceiling of the vehicle and a lower face of the bag lower part is abutted to an upper face of an armrest of a center console.

8. The airbag according to claim 7 , wherein the airbag upper part has an outline shape in a horizontal cross-section in a maximum width portion in the vehicle width direction, the outline shape has a backmost portion in the vicinity of the backmost part of the center console and completely covers the backmost part of the center console in the vehicle width direction in the maximum width portion, the airbag upper part has a reduced width toward the front side so as to form the airbag upper part has a bell shape with a narrow tip end.

9. The airbag according to claim 7 or 8 , wherein the airbag lower part is configured so that a width in the vehicle width direction is wider than the width of the center console in the vehicle width direction, an outline shape in a horizontal cross-section at a position corresponding to the shoulder of the seat back of each of the two seats has a rear part in the vicinity of the backmost part of the center console and a front part in the vicinity of the front end of a seat cushion of the seat so that the rear part and the front part have substantially the same maximum width in the vehicle width direction, and an intermediate portion of the rear part and the front part has a relatively narrow width in the vehicle width direction.

10. An airbag configured to expand between two seats that are arranged in a vehicle in a direction of vehicle width,

wherein the airbag is configured to be installed in a case located behind seating positions of the two seats, and when a gas is provided to the airbag, the airbag is inflated and expanded from the case towards a front side of the vehicle so that an upper end and a lower end of the airbag are positioned on a front side relative to the case,

wherein the airbag has an outline shape in a front view as seen from the front side of the vehicle in an expanded state that is composed of an airbag upper part that forms a part above the height position corresponding to a shoulder of a seat back of the each of the two seats and an airbag lower part that forms a part below the height position corresponding to the shoulder of the seat back of each of the two seats, the airbag upper part has outermost parts expanded up to a vicinity of an inner end of a headrest of each of the seat backs in the vehicle width direction in the front view, the airbag lower part has outermost parts expanded up to a vicinity of respective inner ends of the two seat backs in the vehicle width direction in the front view, and the air bag upper part and the air bag lower part are configured and shaped to be connected to each other in a stepwise manner so as to retain a head, a neck and a chest of the occupant at substantially the same timing when deployed, and

wherein in the expanded state, a cross-section of the airbag lower part that is taken along a substantially horizontal plane located below and adjacent to the height position corresponding to the shoulder of the seat back has a recess to accommodate a shoulder and an arm of the occupant on each side at a position on a front side of the seat backs.