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 (55, 56, 57) 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 (55, 56, 57), and a lens-side mount surface, and wherein said camera body comprises: a plurality of body-side bayonet lugs (25, 26, 27) 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 (28, 29, 30), formed between mutually adjacent body-side bayonet lugs (25, 26, 27) of said body-side bayonet lugs (25, 26, 27), said protrusion being loosely engaged in said recessed portion when each of said lens-side bayonet lugs (55, 56, 57) is positioned in a corresponding one of said circular-arc opening (28, 29, 30), and being positioned at a different circumferential position from said lens-side bayonet lugs (55, 56, 57) 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 (55, 56, 57) are positioned immediately behind said body-side bayonet lugs (25, 26, 27), characterized in that a distance between said protrusion and either of adjacent two body-side bayonet lugs (25, 26, 27) of said body-side bayonet lugs (25, 26, 27) is shorter than the length of each of said lens-side bayonet lugs (55, 56, 57), the number of said body-side bayonet lugs (25, 26, 27) and the number of said lens-side bayonet lugs (55, 56, 57) are three, wherein at least one of said three body-side bayonet lugs (25, 26, 27) is different in length from the other of said three body-side bayonet lugs (25, 26, 27), and said protrusion is positioned in one of three of the circular-arc openings (28, 29, 30) that is the greatest in length. a plurality of body-side bayonet lugs (25, 26, 27) 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 (28, 29, 30), formed between mutually adjacent body-side bayonet lugs (25, 26, 27) of said body-side bayonet lugs (25, 26, 27), said protrusion being loosely engaged in said recessed portion when each of said lens-side bayonet lugs (55, 56, 57) is positioned in a corresponding one of said circular-arc opening (28, 29, 30), and being positioned at a different circumferential position from said lens-side bayonet lugs (55, 56, 57) 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 (55, 56, 57) are positioned immediately behind said body-side bayonet lugs (25, 26, 27), characterized in that a distance between said protrusion and either of adjacent two body-side bayonet lugs (25, 26, 27) of said body-side bayonet lugs (25, 26, 27) is shorter than the length of each of said lens-side bayonet lugs (55, 56, 57), the number of said body-side bayonet lugs (25, 26, 27) and the number of said lens-side bayonet lugs (55, 56, 57) are three, wherein at least one of said three body-side bayonet lugs (25, 26, 27) is different in length from the other of said three body-side bayonet lugs (25, 26, 27), and said protrusion is positioned in one of three of the circular-arc openings (28, 29, 30) that is the greatest in length.