PURPOSE: To wait for operator's intervention in the power-off state of a computer system if a power failure occurs during unmanned operation by providing a condition signal storage device, power failure detecting circuit, and powering- off means.

CONSTITUTION: The computer system 1 is provided with the condition signal storage device, e.g. FF for storing a condition signal which indicates a manned or unmanned operation state, and it is set and reset by an adequate means. A power source controller 2 consists of a power unit, the power failure detecting circuit 4, etc. When the power supply is turned off during the unmanned operation, the computer system 1 is held in a safety state, i.e. in the off state of the power source 3 until next operator's intervention.