PURPOSE: To satisfactorily execute draining, when a rain has stopped, and to easily execute a maintenance by providing a pair of electrodes by placing an insulating part between them in the upper and lower directions of the inclined surface.

CONSTITUTION: A waterdrop sensor 1 has an inclination on the surface of a cylindrical shape, the first electrode 11 and the second electrode 12 are formed in the upper part and the lower part of the surface, respectively, and these first and second electrodes are divided into the upper and lower directions by placing the same circular insulating part 2 between them, and a pair is placed. On each electrode 11, 12, takeoff parts 1a, 1b are provided, connected to a measuring circuit 4 through lead wires 3a, 3b, a resistance between the electrodes 11, 12 is measured, and a waterdrop (raindrop) is detected. According to this waterdrop sensor, the waterdrop drops down and no waterdrop is left, and rubbish, dust, etc. are washed away and scarcely accumulated, a heater, etc. are unnecessary, the titled sensor is small in size, low in cost and has a high reliability, and its maintenance can be executed easily.