PURPOSE: To exactly prevent the occurrence of robbery by a method in which a secret room key consisting of a light receiver, an actuator to operate locking mechanism, and a control circuit is built in a keeping box, and the actuator is operated by a transmitting equipment to emit infrared rays.

CONSTITUTION: A secret room key is made up of an infrared rays transmitter 1, a light receiver 2 attached to the inside of a keeping box, a locking mechanism 3 attached to doors 5 and 6, and a control circuit 4. The mechanism 3 is provided with a coil 10 and a plunger 12, and a sash 5a has an inner cylinder and a sash 6a has a recession 6b, which permits the plunger 12 to go in or out. In unlocking, infrared rays from the transmitter 1 are converted into electrical signals and checked as to whether they are normal marking code signals or not. The coil 10 is electrified to retract the plunger 12 for unlocking. The occurrence of robbery can thus exactly prevented.