PURPOSE: To evade the fatigue of eyes as well as the eyesight deterioration of a computer operator by providing a sensor to a computer display device to detect the position of the operator face.

CONSTITUTION: The ultrasonic waves are radiated from a sensor 10 provided to a computer display device 11. The distance between an operator face and a display screen can be decided by the time during which the ultrasonic wave radiated from the sensor 10 is reflected by the computer face and comes back again to the sensor 10. Then the signal produced by the sensor 10 is transmitted to a control circuit 50 when the distance between the display screen and the operator eyes is set at about 40cm. Thus the circuit 50 rises the warning of danger to the operator via the output 60 of a warning buzzer, by the display 62 of a warning message on the screen, and other methods 64.