Occupant protection system

1. An occupant protection system for protecting an occupant sitting in a seat of a vehicle, the occupant protection system comprising:

a seatbelt that includes a lap belt which for arresting a pelvic region of the occupant;

an inflator; and

an airbag that is formed of a sheet material having flexibility and stored in a storage in the lap belt in a folded form, the airbag being configured to be inflated and deployed upward from the storage when fed with an inflation gas from the inflator,

wherein the airbag includes, when deployed:

a main inflatable portion for protecting an upper body of the occupant; and

a head protecting portion that is located in a vicinity of an upper end of the main inflatable portion for protecting a head of the occupant,

wherein the head protecting portion includes:

a front inflatable portion for covering a front side of the head; and

a pair of side inflatable portions that extend rearward from both ends in a left and right direction of the front inflatable portion for covering sides of the head, each of the side inflatable portions being configured to be deployed in such a manner that a space is formed between itself and the head,

wherein the front inflatable portion is configured to be deployed in such a manner that a space is formed between itself and the head of the occupant,

wherein, at airbag deployment, the side inflatable portions of the head protecting portion extend from the front inflatable portion in such a manner as to draw apart from each other towards leading ends thereof, and are each disposed at a slant with respect to the front inflatable portion at an angle of 30° to 60°,

wherein the head protecting portion further internally includes two tethers each of which connects a central portion in the left and right direction of a front side of the front inflatable portion and a bending point of each of the side inflatable portions from the front inflatable portion in a rear side of the head inflatable portion, and

wherein the head protection portion is formed into a generally board shape having a generally uniform thickness from the front inflatable portion to the side inflatable portions.

2. The occupant protection system of claim 1 , wherein:

the main inflatable portion as fully deployed has a generally triangular prism shape whose axis extends generally in a left and right direction; and

the main inflatable portion as deployed includes, in an underside thereof, a thigh contact surface that is configured to contact with upper surfaces of thighs of the occupant.

3. The occupant protection system of claim 2 , further comprising a pair of thigh arresting portions that protrude downwardly from both ends in the left and right direction of a lower end region of the main inflatable portion at airbag deployment.

4. The occupant protection system of claim 1 , wherein the seatbelt and the inflator are mounted on the seat.

5. An occupant protection system for protecting an occupant sitting in a seat of a vehicle, the occupant protection system comprising:

a seatbelt that includes a lap belt for arresting a pelvic region of the occupant;

an inflator; and

an airbag that is formed of a sheet material having flexibility and stored in a storage in the lap belt in a folded form, the airbag being configured to be inflated and deployed upward from the storage when fed with an inflation gas from the inflator,

wherein the airbag includes, when deployed:

a central inflatable portion for protecting an upper body of the occupant;

a pair of head protecting portions that protrude rearward from left and right end regions of an upper end region of the central inflatable portion for covering sides of a head of the occupant; and

a pair of pelvis protecting portions that protrude rearward and downwardly from left and right end regions of a lower end region of the central inflatable portion for covering sides of a pelvic region of the occupant;

wherein the central inflatable portion as fully deployed has a sufficient width for arresting generally an entirety of the upper body of the occupant in an area between the head protecting portions and pelvis protecting portions; and

wherein the central inflatable portion includes a deformation preventing means for preventing the upper end region of the central inflatable portion from bending forward at receiving the upper body of the occupant.

6. The occupant protection system of claim 5 , wherein the deformation preventing means is constituted by a configuration that the central inflatable portion as deployed includes, in an underside thereof, a supported surface that is supported by thighs of the occupant.

7. The occupant protection system of claim 5 , wherein the deformation preventing means is constituted by a configuration that the central inflatable portion as fully deployed has a generally triangular prism shape whose axis extends generally in a left and right direction, and that a shape of the central inflatable portion as fully deployed is such that two apexes are located one above another in a rear side while a remaining apex is located farther forward than the two apexes, when viewed from a side.

8. The occupant protection system of claim 5 , wherein the seatbelt and the inflator are mounted on the seat.