PURPOSE: To obtain an invariably constant output voltage by attracting or repelling a permanent magnet or magnetic body piece according to the displacement of a rotary disk and stressing the 1st piezoelectric element.

CONSTITUTION: When the projection part of the rotary disk 30 approaches the permanent magnet 12, a positive voltage is developed at the negative electrode of the 1st piezoelectric element 1 and a negative voltage is developed at the positive electrode. No voltage, however, is generated at the 2nd piezoelectric element 21. Then, when the disk 30 rotates and the projection part approaches a recessed part, the generated voltage of this element 11 drops. Thus, a voltage waveform corresponding to projection and recessed parts of teeth of the disk 30 is generated between both terminals of lead wires 13 and 23. Then, when the whole sensor oscillates, the magnet 12 and an iron piece 22 oscillate equally. When the 1st and the 2nd elements 11 and 21 receive a force in elongation direction, voltages generated by the 1st element 11 and the 2nd element 21 cancel each other and no voltage is generated between both terminals of the lead wires 13 and 23. Consequently, the invariably constant output voltage is obtained.