To provide a key control system dispensing with the need for a partici pant who enters unmanned installation to go to the control office for receiving or return a key, and capable of confirming at the control office whether or not the participant enters the unmanned installation, and also capable of signifi cantly reducing the labor for controlling the key of unmanned installation.

The key for opening a door 21 of unmanned installation 2 is kept in a key depository box 11 outside the unmanned installation 2 and a worker entering the unmanned installation 2, for maintenance, inspection, or the like telephones to the unmanned installation 2 through a cellular phone 7 to input through the phone 7 a worker's own identification number previously registered in the communication control device 4 of the unmanned equipment or the unlocking command or the like previously informed by the control office, so that the key can be taken out from the key depository box 11. The communication control device 4 of the unmanned equipment transmits the signal received from the cellular phone 7, a key taking-out signal, or the like to the control office, and these signals are registered by the control office.