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 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, the plurality of second bayonet claw portions (107a, 107b, 107c) includes a first concave portion side bayonet claw portion having at least a portion disposed on the concave portion side, a second concave portion side bayonet claw portion having at least a portion disposed on the concave portion side, and an opposite side bayonet claw portion in which both ends (107a1, 107a2) thereof are provided opposite side of the concave portion side with respect to the boundary line, and θ 1 ≥ θ 2 is satisfied, where θ1 is an angle formed between a line that passes through an end (107c1) of the first concave portion side bayonet claw portion that is closer to the concave portion, and the central axis of the second mount portion, and a line that passes through the center of the concave portion and the central axis of the second mount portion, and θ2 is an angle formed between a line that passes through an end (107b2) of the second concave portion side bayonet claw portion that is closer to the concave portion, and the central axis of the second mount portion, and the line that passes through the center of the concave portion and the central axis of the second mount portion, characterized in that 1.00 ≤ θ 1 / θ 2 ≤ 1.40, and 90 ° < θ 1 + θ 2 < 180 ° 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 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, the plurality of second bayonet claw portions (107a, 107b, 107c) includes a first concave portion side bayonet claw portion having at least a portion disposed on the concave portion side, a second concave portion side bayonet claw portion having at least a portion disposed on the concave portion side, and an opposite side bayonet claw portion in which both ends (107a1, 107a2) thereof are provided opposite side of the concave portion side with respect to the boundary line, and θ 1 ≥ θ 2 is satisfied, where θ1 is an angle formed between a line that passes through an end (107c1) of the first concave portion side bayonet claw portion that is closer to the concave portion, and the central axis of the second mount portion, and a line that passes through the center of the concave portion and the central axis of the second mount portion, and θ2 is an angle formed between a line that passes through an end (107b2) of the second concave portion side bayonet claw portion that is closer to the concave portion, and the central axis of the second mount portion, and the line that passes through the center of the concave portion and the central axis of the second mount portion, characterized in that 1.00 ≤ θ 1 / θ 2 ≤ 1.40, and 90 ° < θ 1 + θ 2 < 180 ° are satisfied.