PURPOSE: To improve the reliability of a computer system by turning off a power source automatically when a diagnosing mechanism which checks the operation of the powered-on computer system at specific interval of time detects abnormality.

CONSTITUTION: When a power-on request is generated, a detection signal (c) is sent to a power source control part 7 and a connection indication signal (b) is sent to a relay 3. A power source 4 supplies electric power to a device 1. A timer 8 generates a signal (e) to an operation monitor mechanism 9 at intervals of time specified previously by the device 1. The operation monitor mechanism 9 receives the signal (e) from the timer 8 and sends an operation monitor signal (g) to the diagnosing mechanism part 2 of the device 1. The diagnosing mechanism part 2 returns a normal operation response signal (h) when the device 1 is in normal operation. If troubles occur to the device 1, the diagnosing mechanism 2 sends an abnormality signal (i) and the operation monitor mechanism 9 sends a power-off request signal l to a request detection part 6 to send a disconnection request signal (d) to the power control part 7. The power source control part 7 ceases a connection request signal (b) in response to the signal and disconnects the power source.