PURPOSE: To obtain a safe rental deposit box by using a double key consisting of a cylinder key as customer key and an operating part as a slidable cover key having a lock pin, an elastic part, and a through hole for the double key.

CONSTITUTION: A double key consists of a cylinder key 2 as a customer key and an operating part to operate the key hole of the key 2. For the operating part, a lock pin 6, a floating spring 7, a through hole 8 larger than the pin 6, and a protect cover 9 are provided to form a slide cover, and the slide cover is attached to a fixing base 4. A coupling piece, a coupling groove, and a slide groove 26 are provided to the lower part of the slide cover in such a way as to enable it to reciprocally move to the longitudinal direction. A key card is inserted into the hole 8, and only when the hole 8 coincides with a given key card, the slide cover is allowed to be coupled or to move. The safety of each deposit box can thus be secured.