To make it possible to detect a water droplet even for opaque material by a constitution that a piezoelectric vibrator is attached to the rear recess surface of a thin part formed by providing a recess part on the rear surface, and that the water droplet is detected by utilizing a resonance phenomenon.

When a driving signal is intermittently outputted from an oscillating circuit 7, the signal is supplied to a piezoelectric vibrator 4 through a resonance circuit 8. A thin part 3 is vibrated at a natural resonance frequency f0. Then, the vibration of the thin part 3 is transduced into the electric signal with the vibrator 4 during the stopped period of the supply of the driving signal, and the circuit 8 is operated. At this time, when the circuit 8 is resonated at the frequency f0, the impedance of the circuit 8 becomes the minimum value, and the current flowing from a positive-side terminal 15 to the circuit 8 and the circuit 7 becomes the maximum value. In the meantime, when a water droplent W is attached on the thin part 3 and the mass of the vibrating system is increased, the vibrating frequency returning to the circuit 8 from the vibrator 4 is displaced. As a result, the impedance of the circuit 8 becomes high, and current of a current detector 14 is decreased. The detected current is monitored by a control part 13. When the decreased amount is larger than a reference value, the attachment of the water droplet W is detected, and a water-droplet remover 16 is operated.