A camera body includes a plurality of body-side bayonet lugs which are provided on the peripheral edge of an opening; and a protrusion which is positioned in a circular-arc opening formed between adjacent body-side bayonet lugs of the body-side bayonet lugs, the distance between the protrusion and either of adjacent two body-side bayonet lugs of the body-side bayonet lugs being shorter than each of the lens-side bayonet lugs; and a body-side mount surface with which the lens-side bayonet surface comes into contact from the front side when the lens-side bayonet lugs are positioned immediately behind the body-side bayonet lugs.