PURPOSE: To stabilize the characteristic of a digital filter in a short time by idly operating a filter operation circuit in certain times at first when a reset signal which makes an action started is generated.

CONSTITUTION: Data which varies temporally is usually converted into digital quantity by an A/D conversion circuit 1 and after executing a low pass filter process by the digital filter 2, it is converted into analog guantity by a D/A conversion circuit 3 and outputted. At the time of initially starting or at the time of resetting the filter a signal is outputted from a reset signal output circuit 7. By outputting a certain value N previously set in a counted value setting circuit 6 to a counter 5 with the reset signal and closing a switch 4 N times, the filter operation is made to be idly operated N times irrespective of an external clock. Therefore, at the time of initially starting or at the time of resetting the filter, the value in last time and the value of filter operation in last time which are used for the filter operation can be set in the optimum values.