To enable the highly accurate detection of raindrops by computing the amount of changes in the amplitude of elastic waves and prforming determination by comparing it with a predetermined value.

A transmitting element 3 and a receiving element 5 are arranged so that the direction of elastic wave transmission of the transmitting element 3 and the direction of reception of the receiving element 5 form a predetermined angle. A voltage of a predetermined frequency is impressed on the receiving element 3 from an oscillator 11, and on the other hand, a preamplifier 13 amplifies a signal voltage converted according to the amplitude of the elastic waves by the receiving element 5 up to such a predetermined signal level as to be signal-processable. Further, signals from a filter circuit 15 are compressed and converted to such a signal level as a microcomputer 21 can conveniently work with. The microcomputer 21 is connected to a log amplifier 17 via an A/D converter 19, and analog signals from the log amplifier 17 are converted into digital signals by the A/D converter 19 and inputted to the microcomputer 21. A wiper control device 23 is connected to the microcomputer 21, and the microcomputer 21 activates windshield wipers in the case that the microcomputer 21 determines that there are raindrops on a windshield 1.