PURPOSE: To obtain a small-sized rainfall sensor which requires no power source by providing a funnel, a tipping bucket, a seesaw arm, a water receiving cup, an air-core coil, a permanent magnet, a light emitting element, a light receiving element, etc.

CONSTITUTION: Rain water which is gathered in the funnel 94 is pooled in the tipping bucket 91 and when the rain wall reaches a constant amount, the measure is turned over to discharge the rain water to the water receiving cup 92. The water reception side moves down quickly and stops at a lower fixed position and the received water flows gradually out of the water discharge port of the bottom part. When the other measure of the tipping bucket 91 reaches the specific amount as rain falls continuously, the measure is turned over similarly and its rain water is discharged to the cup 92 on the other side to slant the seesaw arm 93 in the opposite direction. The permanent magnet 8 is inserted into and extracted from the coil 7 according to the motion of the cup 92 to generate an electromotive force. Consequently, the light emitting element 3 emits light and a rainfall sensor part 1 and a rainfall measurement part 2 are connected by an optical cable 6 through an optical fiber 5; and the light receiving element 4 of the rainfall measurement part 2 receives the light signal and converts it into an electric signal, whose data is integrated to measure the rainfall.