PURPOSE: To detect rain drops simply and accurately.

CONSTITUTION: A light-emitting section 5 and a light-receiving section 6 are disposed oppositely on the boundary section of the bonnet 1 and windshield 2 of an automobile. The light-emitting section 5 receives a synchronizing signal from an amplifier unit 3 stored in a car body and intermittently projects band- shaped light 8, and the quantity of light received, which is obtained by the light-receiving section 6, is output to a control section 4 through the amplifier unit 3. When a rain drop passes between the light-emitting section 5 and the light-receiving section 6, band-shaped light 8 is shielded and the rate of light reception is deteriorated, the rain drop is detected. The controller 4 measures the quantity of the rain drops from the result of the detection, and outputs a wiper driving signal and drives a wiper 7 when the result of the measurement reaches a fixed threshold.