A camera system according to the present invention includes an image pickup device having an imaging surface of an imaging range with a predetermined aspect ratio, the image circle being set larger than the imaging range to include the imaging range therein, a mount portion to which the interchangeable lens is mounted, and an opening portion having an opening for allowing luminous flux from the interchangeable lens to pass through, being located at a position spaced from the imaging surface by a predetermined distance, wherein the size of the opening of the opening portion is equal to or larger than a square formed by connecting ends of segments tilted by an angle .theta. with respect to an optical axis diagonally from four corners of the imaging range and extended to the opening of the opening portion, and the angle .theta. satisfies: 12 degrees.ltoreq..theta., wherein a distance between the opening of the opening portion and the imaging surface, herein referred to as flange back (FB), is: 35 mm.ltoreq.FB.ltoreq.50 mm.