A camera body having a substantially circular opening formed in a front side thereof, wherein a lens barrel being capable of being attached and detached to and from said opening , wherein said lens barrel includes a plurality of lens-side bayonet lugs which are provided at a rear end of said lens barrel and project toward an outer peripheral side, a recessed portion which is formed on an outer edge of one of said lens-side bayonet lugs , and a lens-side mount surface , and wherein said camera body comprises: a plurality of body-side bayonet lugs which are provided on a peripheral edge of said opening and project toward an inner peripheral side; a protrusion which is positioned in a circular-arc opening , formed between mutually adjacent body-side bayonet lugs of said body-side bayonet lugs , said protrusion being loosely engaged in said recessed portion when each of said lens-side bayonet lugs is positioned in a corresponding one of said circular-arc opening , and being positioned at a different circumferential position from said lens-side bayonet lugs which are positioned immediately behind said body-side bayonet lugs, and a body-side mount surface with which said lens-side mount surface comes into contact from a front side when said lens-side bayonet lugs are positioned immediately behind said body-side bayonet lugs , characterized in that a distance between said protrusion and either of adjacent two body-side bayonet lugs of said body-side bayonet lugs is shorter than the length of each of said lens-side bayonet lugs , the number of said body-side bayonet lugs and the number of said lens-side bayonet lugs are three, wherein at least one of said three body-side bayonet lugs is different in length from the other of said three body-side bayonet lugs , and said protrusion is positioned in one of three of the circular-arc openings that is the greatest in length. a plurality of body-side bayonet lugs which are provided on a peripheral edge of said opening and project toward an inner peripheral side; a protrusion which is positioned in a circular-arc opening , formed between mutually adjacent body-side bayonet lugs of said body-side bayonet lugs , said protrusion being loosely engaged in said recessed portion when each of said lens-side bayonet lugs is positioned in a corresponding one of said circular-arc opening , and being positioned at a different circumferential position from said lens-side bayonet lugs which are positioned immediately behind said body-side bayonet lugs, and a body-side mount surface with which said lens-side mount surface comes into contact from a front side when said lens-side bayonet lugs are positioned immediately behind said body-side bayonet lugs , characterized in that a distance between said protrusion and either of adjacent two body-side bayonet lugs of said body-side bayonet lugs is shorter than the length of each of said lens-side bayonet lugs , the number of said body-side bayonet lugs and the number of said lens-side bayonet lugs are three, wherein at least one of said three body-side bayonet lugs is different in length from the other of said three body-side bayonet lugs , and said protrusion is positioned in one of three of the circular-arc openings that is the greatest in length. <EOS>