PURPOSE: To prevent a malfunction caused by a leakage current, etc., by always short-circuiting a bimorph element except at the time of exposing.

CONSTITUTION: A transistor Q2 constitutes a switching means for short-circuiting the bimorph BI in a usual time (when a shutter is not opened or shut) and transistors Q1 and Q3 constitute a switching means for releasing the short-circuit by the transistor Q2 and also charging the bimorph BI only when the shutter is opened. Namely, the transistor Q2 is constituted in a self bias type by a resistance R2 and it becomes in a state where conduction is impossible when the shutter is opened and in a state where conduction is possible when the shutter is shut, so that it discharges the charged charge of the bimorph BI when the shutter is shut and in the usual time. Thus, the leakage current can be surely prevented from flowing in the bimorph element in the usual time and the exposure caused by opening the shutter by mistake can be prevented.