In a transmission mechanism, a motor is driven to rotate a rotation shaft, thereby rotating a rotor cam and a lock gear through a worm gear, a helical gear, and an intermediate gear. A cam is rotated integrally with the lock gear, thereby causing a lock bar to move to a position to restrict rotation of a knob and to a position to release rotation-restriction of the knob. A magnet is provided at the lock gear, and the magnet rotates integrally with the lock gear. Accordingly, the position of the rotor cam and the position of the lock bar can be detected from the position of the magnet.