A connection device formed as a plug, comprising: a rod-shaped electrode ; a plurality of cylindrical electrodes through and into which the rod-shaped electrode passes, said electrodes being sequentially exposed on a surface in order from a front end side of the rod-shaped electrode ; an insulation section configured to insulate the rod-shaped electrode and the plurality of cylindrical electrodes from each other; a plurality of connection terminals each electrically connected to its corresponding electrode from the group of the rod-shaped electrode and the plurality of cylindrical electrodes , such connection terminals being in a vicinity of a rear end of the rod-shaped electrode and protruded from a rear side of the connection device; a conductor pattern (39, 40, 41; 65a, 65b, 65c, 66, 67) on a wiring substrate , the plurality of connection terminals being connected to the conductor pattern (39, 40, 41; 65a, 65b, 65c, 66, 67) and arranged to establish a connection with an external unit through the wiring substrate ; characterized by a substantially cylindrical extension electrode extending between one of the plurality of cylindrical electrodes , said cylindrical electrodes being ring-shaped, and one of the plurality of connection terminals , wherein the substantially cylindrical extension electrode has a diameter smaller than that of the ring-shaped cylindrical electrode , wherein the substantially cylindrical extension electrode is fixed to the ring-shaped cylindrical electrode , and wherein the rear side end of the substantially cylindrical extension electrode is thin plate shaped and protrudes from a predetermined position of the rear side , said rear side being formed as a rear collar , said electrode protruding as one of the plurality of connection terminals , forming a contact. a rod-shaped electrode ; a plurality of cylindrical electrodes through and into which the rod-shaped electrode passes, said electrodes being sequentially exposed on a surface in order from a front end side of the rod-shaped electrode ; an insulation section configured to insulate the rod-shaped electrode and the plurality of cylindrical electrodes from each other; a plurality of connection terminals each electrically connected to its corresponding electrode from the group of the rod-shaped electrode and the plurality of cylindrical electrodes , such connection terminals being in a vicinity of a rear end of the rod-shaped electrode and protruded from a rear side of the connection device; a conductor pattern (39, 40, 41; 65a, 65b, 65c, 66, 67) on a wiring substrate , the plurality of connection terminals being connected to the conductor pattern (39, 40, 41; 65a, 65b, 65c, 66, 67) and arranged to establish a connection with an external unit through the wiring substrate ; characterized by a substantially cylindrical extension electrode extending between one of the plurality of cylindrical electrodes , said cylindrical electrodes being ring-shaped, and one of the plurality of connection terminals , wherein the substantially cylindrical extension electrode has a diameter smaller than that of the ring-shaped cylindrical electrode , wherein the substantially cylindrical extension electrode is fixed to the ring-shaped cylindrical electrode , and wherein the rear side end of the substantially cylindrical extension electrode is thin plate shaped and protrudes from a predetermined position of the rear side , said rear side being formed as a rear collar , said electrode protruding as one of the plurality of connection terminals , forming a contact. <EOS>