A seat backrest includes right and left side frame segments, a pressure receiving member disposed between the side frame segments movably in a front-back direction, and a cushion pad covering the front of the side frame segments and pressure receiving member. The pressure receiving member includes a main body part, and extending parts extending laterally from both sides of the main body part. The cushion pad includes a back receiving part covering the front of the main body part, and bank parts disposed on both sides of the back receiving part, protruding forward from both sides, and covering the front of the extending parts and side frame segments. A recess is formed at an area, facing a corresponding extending part of the extending parts, of a rear surface of each bank part.