PURPOSE: To ensure locking while preventing unjust release by installing a locking member engaged or detached with or from the engaging section of an external sliding door and the driving member of the locking member to an internal sliding door and mounting an operating piece for locking and an operating piece for release to the external sliding door.

CONSTITUTION: A locking member 48 engaged or detached with or from the engaging section 42 of an external sliding door A and the driving member 50 of the locking member 48 are set up to an internal sliding door B, and an operating piece 25 for locking and an operating piece 26 for release are fitted to the external sliding door A. When a key 6 is inserted into a key hole formed to the external sliding door A and turned in the direction of locking, the driving member 50 installed to the internal sliding door B is operated through the operating piece 48 for locking, the locking member 48 is rotated and engaged with the engaging section 42 of the external sliding door A, and the stage of locking is brought. When the key 6 is revolved in the direction of release, the driving member 50 is worked in the direction of release through the operating piece 26 for release, the locking member 48 is detached from the engaging section 42 of the external sliding door A, and the state of release is brought.