PURPOSE: To eliminate a mistake when the using state of a terminal device is checked by reading out the value of a counter for every prescribed period to compare this value with a prescribed reference level and deciding the terminal device is not used when the value of the counter is larger than the reference level.

CONSTITUTION: A host computer 1 outputs an inquiry to a terminal 2 through a circuit, and a timer 5 is reset if it is decided by a detecting circuit 4 that a maximum value detecting circuit 11 of the terminal 2 does not reach the maximum value. Then the timer 5 starts counting again. An interruption is applied to the terminal 2 after the counted value of the timer 5 reaches a prescribed level. A counter 8 of the terminal 2 is set at zero for each key input and therefore shows the small value when the terminal 2 is in use. When the circuit 11 reaches the maximum value, the circuit 4 shows the coincidence. This coincidence is informed to a monitor circuit 6 to indicate a power supply breaking circuit 7 from the circuit 6 to turn off the power supply of the terminal 2.