Provided is a plug wherein a plurality of points in an electric circuit can be easily put into a conductive state or a nonconductive state. A system wherein such a plug is applied is also provided. The composite plug is provided with a plurality of plugs, each of which is provided with two conductive pieces connected by a conductive member. A current is carried between the two circuit terminals when the two conductive pieces are brought into contact with two circuit terminals included in the electric circuit, respectively. A current is not carried between the two circuit terminals when the two conductive pieces are separated from the two circuit terminals included in the electric circuit, respectively. The plugs are attached to a common case body.