In a mount device including a mount which an accessory is attachable to and detachable from, a plurality of terminals arranged along a circumferential direction of the mount and used for electrical connection with the accessory includes a first terminal which is used to supply driving electric power serving as a first voltage to the accessory, and a second terminal which exhibits a voltage that changes according to the accessory mounted to the mount device, wherein the second terminal is able to be set to an input state or an output state, and is set to the input state during a period in which the driving electric power is not output to the accessory via the first terminal.