Accessory, image pickup apparatus on which same is mountable, and camera system

An accessory that is detachably mountable to an image pickup apparatus including a first mount portion and a pin, the first mount portion including a plurality of first bayonet claw portions, the accessory comprising:

• a second mount portion that includes a plurality of second bayonet claw portions configured to be engaged with the plurality of first bayonet claw portions, and a recess into which the pin is inserted when the accessory is mounted to the image pickup apparatus;
• an accessory body;
• and
• a mount fixing member for fixing the second mount portion to the accessory body, and, when, viewed in a central axis direction of the second mount portion, a recess side is a side on which the recess 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 recess and a central axis of the second mount portion and that passes through the central axis of the second mount portion, and when, among the plurality of second bayonet claw portions, two second bayonet claw portions at least a part of which is on the recess side are denoted as a first recess side bayonet claw portion and a second recess side bayonet claw portion,
• 90°<θ1+θ2<180°
• is satisfied, where θ1 is an angle formed between a line that passes through the central axis of the second mount portion and an end of the first recess side bayonet claw portion, the end being close to the recess, and a line that passes through the center of the recess and the central axis of the second mount portion, and θ2 is an angle formed between a line that passes through the central axis of the second mount portion and an end of the second recess side bayonet claw portion, the end being close to the recess, and a line that passes through the center of the recess and the central axis of the second mount portion, and the recess is disposed in a region including a cutaway portion between the first recess side bayonet claw portion and the second recess side bayonet claw portion.