A locking device for a seat back includes a release preventing mechanism for preventing the locking device from being unintendedly released by the fact that a second arm portion of a lift lever abuts on a side edge portion of a guide hole of a lock plate to cause the lift lever to rotate positively when a base plate rotates forward along a horizontal direction about a lock pin in a state where the lock pin engages with a lock hole of the lock plate and the second arm portion of the lift lever is located in the guide hole of the lock plate when the seat back is held at a rising position.