An intermittent wiper system with a raindrop sensor has been developed which detects the amount of rain falling and automatically adjusts the intermittent wiper interval.Raindrop detection is accomplished by converting the kinetic energy of raindrops striking a vibrating plate into an electric voltage. The sensor unit consists of the vibrating plate, a piezoelectric element and a signal amplifying circuit. In order to ensure that the output signal would always be proportional to the amount of rain striking the windshield, various analyses were done to clarify what effect the shape of the raindrop sensor, place of installation and attachment angle would have on detection sensitivity. Methods were also investigated and adopted for eliminating the influence of undersirable vibration noise generated by the vehicle horn or when driving on rough roads. The results obtained made it possible to achieve a system constructed of simple parts that controls the intermittent wiper operation to the optimum interval so as to match the amount of rain falling and the vehicle speed.