A seat apparatus includes a lower arm for attachment to a seat-cushion, an upper arm for attachment to a seat-back, a shaft mounted to the upper arm and supported rotatably by the lower arm, a locking mechanism disposed between the lower arm and the upper arm and having a pawl device and a ratchet which are in meshing engagement with each other, and a cam for establishing an engagement between the pawl device and the ratchet when the cam is rotated in one direction and brought into engagement with the pawl device, establishing a disengagement between the pawl device and the ratchet when the cam is rotated in the other direction and disengaged from the pawl device, the cam being mounted on the shaft such that the cam is rotatable with the shaft but movable toward the pawl device when the cam engages with pawl device.