An accessory (100; 40; 70) configured to be detachably mountable to an image pickup apparatus that includes a first mount portion and a pin, the first mount portion including a plurality of first bayonet claw portions (208a, 208b, 208c), the accessory (100; 40; 70) comprising: a second mount portion that includes a plurality of second bayonet claw portions (107a, 107b, 107c) configured to enable engagement with the plurality of first bayonet claw portions (208a, 208b, 208c), and a concave portion in which the pin is inserted when the accessory (100; 40; 70) is mounted to the image pickup apparatus, wherein, when viewed in a central axis direction of the second mount portion, a concave portion side of the second mount portion is a side on which the concave portion is provided with respect to a boundary line, the boundary line being a line that is orthogonal to a line passing through a center of the concave portion and a central axis of the second mount portion and that passes through the central axis of the second mount portion, among the plurality of second bayonet claw portions (107a, 107b, 107c), two second bayonet claw portions that have at least a portion disposed on the concave portion side are denoted as a first concave portion side bayonet claw portion and a second concave portion side bayonet claw portion, the plurality of second bayonet claw portions (107a, 107b, 107c) include the first concave portion side bayonet claw portion, the second concave portion side bayonet claw portion, and an opposite side bayonet claw portion in which both ends (107a1, 107a2) thereof are provided on opposite side of the concave portion side with respect to the boundary line, and θ 10 + θ 20 + θ 30 + θ ⁢ L = 360 ° is satisfied, where, among internal angles of a quadrangle formed by connecting a center (107a3) of the opposite side bayonet claw portion, a center (107b3) of the second concave portion side bayonet claw portion, a center (107c3) of the first concave portion side bayonet claw portion, and the center of the concave portion, θ10 is a first internal angle in which a vertex thereof is the center (107a3) of the opposite side bayonet claw portion, θ20 is a second internal angle in which a vertex thereof is the center (107b3) of the second concave portion side bayonet claw portion, θ30 is a third internal angle in which a vertex thereof is the center (107c3) of the first concave portion side bayonet claw portion, and θL is a fourth internal angle in which a vertex thereof is the center of the concave portion, characterized in that 75° ≤ θ10 ≤ 105°, 75° ≤ θ20 ≤ 105°, 75° ≤ θ30 ≤ 105° and 75° ≤ θL ≤ 105° are satisfied. a second mount portion that includes a plurality of second bayonet claw portions (107a, 107b, 107c) configured to enable engagement with the plurality of first bayonet claw portions (208a, 208b, 208c), and a concave portion in which the pin is inserted when the accessory (100; 40; 70) is mounted to the image pickup apparatus, wherein, when viewed in a central axis direction of the second mount portion, a concave portion side of the second mount portion is a side on which the concave portion is provided with respect to a boundary line, the boundary line being a line that is orthogonal to a line passing through a center of the concave portion and a central axis of the second mount portion and that passes through the central axis of the second mount portion, among the plurality of second bayonet claw portions (107a, 107b, 107c), two second bayonet claw portions that have at least a portion disposed on the concave portion side are denoted as a first concave portion side bayonet claw portion and a second concave portion side bayonet claw portion, the plurality of second bayonet claw portions (107a, 107b, 107c) include the first concave portion side bayonet claw portion, the second concave portion side bayonet claw portion, and an opposite side bayonet claw portion in which both ends (107a1, 107a2) thereof are provided on opposite side of the concave portion side with respect to the boundary line, and θ 10 + θ 20 + θ 30 + θ ⁢ L = 360 ° is satisfied, where, among internal angles of a quadrangle formed by connecting a center (107a3) of the opposite side bayonet claw portion, a center (107b3) of the second concave portion side bayonet claw portion, a center (107c3) of the first concave portion side bayonet claw portion, and the center of the concave portion, θ10 is a first internal angle in which a vertex thereof is the center (107a3) of the opposite side bayonet claw portion, θ20 is a second internal angle in which a vertex thereof is the center (107b3) of the second concave portion side bayonet claw portion, θ30 is a third internal angle in which a vertex thereof is the center (107c3) of the first concave portion side bayonet claw portion, and θL is a fourth internal angle in which a vertex thereof is the center of the concave portion, characterized in that 75° ≤ θ10 ≤ 105°, 75° ≤ θ20 ≤ 105°, 75° ≤ θ30 ≤ 105° and 75° ≤ θL ≤ 105° are satisfied.