A position Z of an image sensor is moved to an initial position A (image forming position) in a Z axis direction. An exposing operation is performed at a position A, and it is determined whether or not an exposing time counted by a timer has reached ta. If the exposing time has reached ta, an image sensor 27 is moved to a position B. The exposing operation is performed at the position, and it is determined whether or not a total exposing time has reached ta+tb. If the total exposing time t has reached ta+tb, the image sensor is moved to a position C, and the exposing operation is performed at the position. Thus, the output distribution of the image sensor can be assigned a desired low pass filter characteristic.