Abstract: After the initiation of a first current-supplying time an insulator of a coated wired W1 cuts off an electrically conductive path and hence a secondary current I2 for melting and removing the insulator is caused to flow from an upper electrode 20 by way of an intermediate electrode 26. An interior conductor is exposed to form an electrically conductive path leading to workpieces (W1 and W2) between the upper electrode 20 and a lower electrode 22, allowing a branch current IB to flow into the lower electrode 22. A control unit monitors a current detection signal <IB> from a current detecting circuit 34. When the signal <IB> has reached a skip level SK, the control unit terminates the supply of current of a first current-supplying time and initiates the current supply of a second current-supplying time for joining the wire W1 to a terminal member W2.

Abstract: A current sensor 20 in the form of a coil such as a toroidal coil provides as its output a current sense signal TS representative of a differential waveform of a welding current I. In a waveform restoring unit 22, integrating circuits 30 and 32 are switchable between an enable state (first state) and a reset state (second state). In the enable state, the current sense signal TS from the current sensor 20 is integrated and issued as integration value signals AIa and AIb representative of a waveform of the welding current I, whereas in the reset state, the integration value signals AIa and AIb are reset to reference values (e.g., 0 volt). A switching circuit 34 detects timings of each cycle or each half-cycle of the welding current I on the basis of a waveform of the current sense signal TS, and alternately switches the states of the integrating circuits 30 and 32 in a complementary manner and at a predetermined timing so that while one is placed in the enable state, the other is in the reset state.