In a back portion of a seat back provided with a lumbar support portion 20 and a side support portion 21, a seat back cover 30 for covering a back face of the seat back is provided. The seat back cover 30 is provided with a side portion 32 for covering an air bag module 50 by making a forward end portion, which is extended to the front side of a vehicle, come into contact with the side support portion 21. The side support portion 21 is provided being capable of oscillating to the front side of the vehicle with respect to the lumbar support 20. An air bag module 50 is arranged in an accommodation space 90 provided on the rear side of the side support portion 21.