A reclining device for a seat comprises a base plate secured to a seat cushion; and a pivotal plate pivotally connected through a center shaft to the base plate and secured to a seatback. The base plate has a guide portion which faces the pivotal plate. A toothed member is movably guided by the guide portion. The toothed member has a first toothed portion. A cam member is movably guided by the guide portion of the base plate. The cam member moves the toothed member in a first direction when moved in one direction and moves the toothed member in a second direction when moved in the other direction. The pivotal plate has a recess which faces the guide portion of the base plate. The recess has a second toothed portion to which the first toothed portion is engaged when the toothed member is moved in the first direction. An operation lever is pivotally connected through the center shaft to the base plate. A mechanism is used for moving the cam member in the one and the other directions when the operation lever is manipulated.