An optional device of the present disclosure is attachable and detachable with respect to an image forming apparatus via a connection member fitted to a side face of a main body of the optional device. The connection member includes a shaft, a connector, and a lock member. The shaft is supported by the main body to be slidable in an axial direction. The connector is fixed to the shaft and coupled to the image forming apparatus. The lock member is capable or restricting movement of the shaft in the axial direction. A position of the connector is adjusted by releasing the restriction placed on the shaft by the lock member and then sliding the shaft in the axial direction.