A mirror box comprises a quick return mirror and an optical low-pass filter. When observing a subject image, the quick return mirror is set to a reflecting position, in which the quick return to mirror is inclined to the optical axis of a photographing lens group by approximately 45 degrees so that incident light is reflected to a view-finder. The optical low-pass filter is set to a non-functioning position, in which the optical low-pass filter is inclined to be parallel to the quick return mirror, and is close to the quick return mirror. When carrying out a photographing operation, the quick return mirror is retracted from the optical axis, and is set to a retracted position, which is parallel to the optical axis. The optical low-pass filter is set to a functioning position which is perpendicular to the optical axis.