This seat of a vehicle is provided with: a center supporting block having a first supporting surface for supporting an occupant; side supporting blocks that are arranged at both sides in the width of the center supporting block, and having a second supporting surface that more protrude than the first supporting surface of the center supporting block; movable supporting blocks each arranged between the center supporting block and the side supporting blocks, and each having a third supporting surface which is flush with the first supporting surface while in an initial state; joining portions each joining between the side supporting blocks and the movable supporting block; and expanding devices that are arranged at a back side of the side supporting blocks, expand the side supporting blocks, and incline the movable supporting blocks with respect to the first supporting surface of the center supporting block.