To prevent inflow of a primary side current to a secondary side when a surrounding temperature is increased or the secondary side is open or short- circuited, by method wherein a positive characteristic thermistor connecting in series to the primary side of a piezoelectric transducer element is provided.

A positive characteristic thermistor element 33 is in series connected to a primary side of a piezoelectric transducer element 6, and further to a power supply 2. If a temperature is increased, a resistance value is increased in characteristics of the positive characteristic thermistor element 22. If a voltage of a half-wave sine wave is inputted from a power supply 2 to the primary side of the piezoelectric transducer element 6 via the positive characteristic thermistor element 33, an output voltage is caused between an output electrode 13 and a common electrode on a secondary side. If the surrounding temperature of the piezoelectric transducer element 6 is increased, primary side impedance thereof is decreased, but the resistance value of the positive characteristic thermistor element 33 is increased, so that the increase in a primary side current i is prevented.