Sliding of a seat or reclining of a seat back can be disabled, except when a rider intends to slide the seat or recline the seat back. The motion of an auxiliary bar which permits sliding of a seat is inhibited by an interlocking lock member. An operating piece cannot be raised and hence it is impossible to slide the seat. An engine key is inserted into a first lock mechanism and is turned, causing the interlocking lock member to slide up to a position permitting operation of the operating piece. In addition, a lever of a second lock mechanism faces a reclining lever, so that the reclining lever cannot turn clockwise around a rivet bolt and a pawl remains engaged with any of first to third recesses. If the lever of the second lock mechanism is turned, the reclining lever can be pulled to release the reclining locked condition.