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 half lines that extends from a central axis of the second mount portion and passes through second terminals that are used in communication between the image pickup apparatus and the accessory, all pass through a predetermined second bayonet claw portion.