The coaxial connector 10 to be connected with a coaxial cable has a surface covering layer 20 formed of a metal material which causes an eutectic reaction with In on the surface of a terminal 12 which is a core conductor. Because of the surface covering layer 20 of a metal material which causes the eutectic reaction with In formed on the surface of the terminal of the coaxial connector, the terminal of the coaxial connector and the electrode of a superconductor device can be jointed to each other by the solder layer of In-based solder at a relatively low temperature. Thus, the discharge of oxygen from the inside of the superconductor film of a superconductor filter can be suppressed, and the decrease of the critical temperature TC can be suppressed.