Maximum and minimum aperture sizes are regulated with regard to a diaphragm in order to secure predetermined optical capability in an optical unit which includes a taking lens and the diaphragm. In a normal taking, the diaphragm is used within a normal taking range from the maximum to the minimum aperture sizes. In the present invention, an aperture size which is larger than the maximum size and a size which is smaller than the minimum size (extra aperture size or extra small size) are respectively set at outside the range that secures the capability. The aperture sizes of the diaphragm at outside the specified range are used at least for one of the following: automatic exposure (AE) adjustment, auto focus (AF) adjustment, electronic zoom, displaying a moving image, taking for recording the moving image, and taking under a low resolution by thinning out pixels.