A connection device formed as a plug, comprising: a rod-shaped electrode; a plurality of cylindrical electrodes (12, 13, 14) through and into which the rod-shaped electrode passes, said electrodes (12, 13, 14) being sequentially exposed on a surface in order from a front end side of the rod-shaped electrode; an insulation section (15, 16, 17) configured to insulate the rod-shaped electrode and the plurality of cylindrical electrodes (12, 13, 14) from each other; a plurality of connection terminals (21, 22, 23, 24) each electrically connected to its corresponding electrode from the group of the rod-shaped electrode and the plurality of cylindrical electrodes (12, 13, 14), such connection terminals (21, 22, 23, 24) 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 (31, 61), the plurality of connection terminals (21, 22, 23, 24) 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 (31, 61); characterized by a substantially cylindrical extension electrode extending between one of the plurality of cylindrical electrodes (12, 13), said cylindrical electrodes (12, 13) 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 (12, 13, 14) through and into which the rod-shaped electrode passes, said electrodes (12, 13, 14) being sequentially exposed on a surface in order from a front end side of the rod-shaped electrode; an insulation section (15, 16, 17) configured to insulate the rod-shaped electrode and the plurality of cylindrical electrodes (12, 13, 14) from each other; a plurality of connection terminals (21, 22, 23, 24) each electrically connected to its corresponding electrode from the group of the rod-shaped electrode and the plurality of cylindrical electrodes (12, 13, 14), such connection terminals (21, 22, 23, 24) 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 (31, 61), the plurality of connection terminals (21, 22, 23, 24) 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 (31, 61); characterized by a substantially cylindrical extension electrode extending between one of the plurality of cylindrical electrodes (12, 13), said cylindrical electrodes (12, 13) 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.