PURPOSE:To connect a test device and test a circuit regardless of the situation whether the circuit is in use or not by providing capacitors between the first and the second plugs, and connecting a test terminal to the second plug. CONSTITUTION:A capacitor C2 is connected between plugs 61-1 and 61-2 and a capacitor C1 between plugs 61-3 and 61-4 respectively, jacks 62-1 and 62-2 are connected to one-side plugs, e.g. plugs 61-2 and 61-4, respectively, and a test device 40 is connected via jacks 62-1 and 62-2. Capacitors C1 and C2 electrically interrupt a circuit 70 and a switched network 90 so that no test signal is fed to the switched network 90. The circuit 70 can be tested without being once cut off, and the circuit 70 can be tested regardless of the situation whether it is in use or not.
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