When a camera body is shaken, a piezoelectric element generates a signal corresponding to angular velocity. The output signal of the element is amplified by a synchronous detection amplifier and is sent to a high-pass filter. An output signal of the filter is integrated after digital conversion and is outputted as a shake amount of the camera body. An initialization-trigger generating circuit generates an initialization trigger in predetermined cycles. When the trigger is inputted, the amplifier outputs a first zero-level signal. A zero-level initialization circuit initializes the filter every input of the trigger to output a second zero-level signal. The two zero-level signals have a reference signal value to be outputted at a time of the angular velocity zero. During the initialization, the reference signal value is applied to both sides of the filter to remove drift components included in the output signal thereof.