A seat back locking device for an automobile for a seat back 5 configured so as to be capable of being changed from an upright position to a folded-down position and vice versa, including a locking mechanism 10 for locking the seat back 5 in the upright position, a locking knob 20 for unlocking the locking mechanism 10, and a key cylinder 30 capable of locking and unlocking the locking knob 20. In the locking device, a contact part 33 that comes into contact with a contacted part 26 provided on the locking knob 20 is connectingly provided integrally with a cylinder body 31 of the key cylinder 30 so that the key cylinder 30 can be changed over between a locked state in which the contact part 33 comes into contact with the contacted part 26 to inhibit the rotation of the locking knob 20 and an unlocked state in which the contact part 33 does not come into contact with the contacted part 26 to allow the rotation of the locking knob 20.