A modular unit removably mounted to a main body includes a cover openably closable relative to a unit body of the modular unit and a movable member to move to the main body in conjunction with closing of the cover. As the cover is closed with the modular unit mounted to the main body, a projection provided to the movable member pushes a lever provided to the main body, and a detector provided to the main body detects that the lever is pushed by the projection of the movable member.