PURPOSE: To easily and securely display a photodetection level.

CONSTITUTION: The photodetection level detected by a photodetecting means R is supplied to a display control part 11 and a transmitter 13. The display control part 11 finds a blinking frequency corresponding to the supplied photodetection level and makes an LED 12 blink at the found frequency. The photodetection level sent from the transmitter 13 is demodulated and decoded by a receiver 4 and supplied to a display control part 3. Consequently, the display control part 3 finds the blinking frequency corresponding to the photodetection level and makes an LED 2 blink at the found frequency. An operator, therefore, only adjust the up-down and right-left angles of the projection optical system of a projecting means T so as to decrease the blinking frequency of the LED 2 eventually to a lights-out state and also only adjust the up-down and right-left angles of the photodetection system of the photodetecting means R so as to decrease the blinking frequency of the LED 12 eventually to the lights-out state.