A side airbag unit disposed in a side support of a seat capable of absorbing collision energy sufficiently during a side collision of a vehicle is described. In one example, the airbag unit comprises: a low expansion member disposed inside of a front skin of the seat, wherein one end of the low expansion member is attached to the inside of the front skin in the vicinity of a fragile part of the seat and the other end of the low expansion member is attached to the seatback frame through a slit. The low expansion member inhibits the front skin from expanding due to the inflation of the airbag, and a cushion pad located outside of the slit in the vehicle width direction and inside of the fragile part in the vehicle width direction is moved outward and frontward by the inflation of the airbag in a vehicle collision.