To reduce fine adjustment and secure stable displacement, by a method wherein resonance frequencies of an adhered piezoelectric element and a member enlarging displacement are set at specific values, and a top end of the member enlarging displacement is bent at a specific angle in a direction of keeping away the adhered piezoelectric element from an adhered portion to provide a shielding part.

An elastic plate is bent in a rectangular, and a shim 11 and a displacement enlarging part 13 are integrally formed. A top end for a joining part of the displacement enlarging part 13 is bent at 90 degrees or less as a bending angle 24 to the opposite side to the shim 11 to form a shielding part 21. The shim 11 is interposed between a sensor pedestal 14 and a fixing member 15, and fixed by fixing screws 16a, 16b. An infrared rays detection part 19 and a thermistor 22 are disposed in the sensor pedestal 14. An adhered piezoelectric element (element) 12 is attached to the shim 11. A difference between resonance frequencies of the element 12 and displacement enlarging part 13 is set within 40%. If the element 12 is oscillated, the displacement enlarging part 13 is displaced and the shielding part 21 constitutes chopper opening/closing infrared rays 20.