To discriminate an unlocked state or locked state with the thumb-turn direction from indoor side by locking easily and simply only by locking operation of one cylinder during locking, by simplifying unlocking operation and improving anti-theft effect by requiring unlocking operation to two cylinders during unlocking, and also obtaining a good feeling of operation during locking and unlocking operation for a dead bolt by the thumb-turn.

Locking and unlocking are performed by projecting or retreating a dead bolt 2 by the operation of a thumb-turn according to the configuration, when locking by a cylinder, the dead bolt 2 is projected out for locking by the operation of only one cylinder among two cylinders 5, 6, when unlocking by the cylinder, either one of cylinders 5 and 6 is operated and then another cylinder 6 or 5 is operated thereby retreating the dead bolt 2 and then unlocking can be achieved.