An accessory (100) configured to be detachably mountable to an image pickup apparatus (10) including a first mount portion (201, A) including first bayonet claw portions (201a-201c), and first terminals (203a-203k), the accessory including a second mount portion (301, B) including second bayonet claw portions (301d-301f) configured to enable engagement with the first bayonet claw portions, and second terminals (303a-303k, 303m) configured to enable contact with the first terminals, in which the second terminals are provided at positions that are different from positions of the second bayonet claw portions, wherein a half line that extends from a central axis of the second mount portion and passes through a second terminal that determines whether an optical apparatus is mounted on the image pickup apparatus or not, passes through a predetermined second bayonet claw portion.