An embodiment airbag for a seat belt includes an airbag cushion deployable from the seat belt, a first support having a shape that widens toward a space in front of a passenger at a lower portion of the airbag cushion when the airbag cushion is in a deployed state, wherein the first support is configured to be supported by thighs of the passenger, and a second support having a shape that widens toward the passenger at an upper portion of the airbag cushion when the airbag cushion is in the deployed state, wherein the second support is configured to support a head of the passenger.