PURPOSE: To protect an obstacle such as the knee of a passanger sitting on a rear seat by a method wherein an obstacle sensor is provided on the back surface of a seat back, which detects a contact with the obstacle and prevents a rearward rotation of the seat back.

CONSTITUTION: The reclining device comprises a base plate 2 attached to the side of a sitting part 1, an arm 5 attached to the side of the seat back 4 and pivoted to a shaft 3 of the base plate 2, a ratchet 6 provided opposing the lower end to a tooth groove parts 2a of the base plate 2, and the sensor plate 7. The upper end of the sensor plate 7 is pivoted to the back surface of the seat back 4 and the lower part thereof is biased in the counterclockwise direction by a spring 9. When the sensor plate 7 contacts any obstacle such as the knee 14 of the passanger sitting on the rear seat, the lower end of the sensor plate 7 touches the ratchet 6, and the ratchet 6 comes into an engagement with the tooth groove part 2a of the base plate 2, thereby preventing the rearward rotation of the seat back 4. This sensor plate 7 can be replaced by any other suitable sensor means which detects the contact with the obstacle and provides a signal to operate the seat back engaging means.