An airbag unit includes a seat, a pair of side airbags, a seat state detector, and an airbag deployment controller. The seat is reversely rotatable in a front-rear direction of a vehicle. The pair of side airbags are expansive and deployable, and incorporated in the seat on respective sides of the seat in a width direction of the vehicle. The seat state detector detects a rearward-oriented state of the seat, i.e., a state in which the seat is rearward-oriented in the front-rear direction. The airbag deployment controller performs a deployment control of the pair of side airbags, on a condition that the seat state detector detects the rearward-oriented state of the seat. The airbag deployment controller performs the deployment control in a reverse sequence from a deployment control in a frontward-oriented state of the seat, i.e., a state in which the seat is frontward-oriented in the front-rear direction.