A seat carriage is mounted on a fixed support in such a manner as to be rotatable clockwise and counterclockwise about a first rotation center. An engagement member is mounted on the fixed support to restrict the rotation of the seat carriage within a range of about 180 degrees. A transmission member can rotate clockwise and counterclockwise about the first rotation center. A pushing member is mounted for rotation about the first rotation center. The location of the pushing member is on a circle along which the transmission member moves and outside a range in which the transmission member moves for rotating the seat carriage over the 180 degree range. A motor causes the pushing member to rotate about the first rotation center and also bring the pushing member into contact with the transmission member for rotation therewith. The transmission member is also rotatable about a second rotation center so that the contact between the pushing member and the transmission member can be removed. The contact between the pushing member and the transmission member is maintained by a spring until the further rotation of the seat carriage is prevented by the engagement member. <IMAGE>
