Airbag

1. An airbag comprising:

a front panel; and a rear panel, outer peripheries of the front panel and the rear panel being joined together, wherein

each of the front panel and the rear panel has a propeller shape in which a plurality of blade portions extend radially from a center portion such that recessed portions are formed between adjacent blade portions in plan view before the front panel and the rear panel are joined, and

the plurality of blade portions of the front panel are joined to corresponding recessed portions of the rear panel, and the plurality of blade portions of the rear panel are joined to corresponding recessed portions of the front panel.

2. The airbag according to claim 1 , wherein a maximum length in a radial direction of each of the blade portions is greater than a maximum length in a width direction perpendicular to the radial direction.

3. The airbag according to claim 2 , wherein each of the blade portions includes a middle portion whose length in the width direction perpendicular to the radial direction becomes maximum, and also includes a tip portion in the radial direction.

4. The airbag according to claim 3 , wherein a length in the width direction of the middle portion of each of the blade portions is greater than a length in the width direction of a portion where each of the blade portions contacts the center portion.

5. The airbag according to claim 3 , wherein the middle portion includes outer edge portions that are bent so as to project in the width direction perpendicular to the radial direction.

6. The airbag according to claim 5 , wherein the front panel and the rear panel are joined together such that the outer edge portions of the front panel coincide with the outer edge portions of the rear panel.

7. The airbag according to claim 3 , wherein a length from a middle portion to a tip portion is greater than a length from a center portion to the middle portion of each of the blade portions of the front panel, and

a length from a middle portion to a tip portion is less than a length from a center portion to the middle portion of each of the blade portions of the rear panel.

8. The airbag according to claim 3 , wherein a length from a middle portion to a tip portion is less than a length from a center portion to the middle portion of each of the blade portions of the front panel, and

a length from a middle portion to a tip portion is greater than a length from a center portion to the middle portion of each of the blade portions of the rear panel.

9. The airbag according to claim 3 , wherein a length from a middle portion to a tip portion is equal to a length from a center portion to the middle portion of each of the blade portions of the front panel, and

a length from a middle portion to a tip portion is equal to a length from a center portion to the middle portion of each of the blade portions of the rear panel.

10. The airbag according to claim 3 , wherein the rear panel includes at least one vent hole on a side closer to a center portion than to a middle portion.

11. The airbag according to claim 3 , wherein the front panel includes at least one vent hole on a side closer to a tip portion than to a middle portion.

12. The airbag according to claim 1 , wherein the outer peripheries of the front panel and the rear panel have a same peripheral length, and are joined together.

13. The airbag according to claim 1 , wherein the plurality of blade portions of the front panel have a same outer peripheral shape, and the plurality of blade portions of the rear panel have a same outer peripheral shape.