PURPOSE: To determine continuously the instantaneous value of electric power to be applied to a weld zone during welding in an automatic controlling method for welding by the measurement of high frequency current, voltage, etc., by calculating the proportional constant during high-frequency butt seam welding.

CONSTITUTION: The primary voltage E1 and current I1 of a current transformer 9 of a high-frequency oscillation circuit are measured and are inputted to a synchroscope 1, the Lissajour's waveform thereof determines the voltage 2V1 in the part cut by the Y-axis. The phase difference between the voltage E1 and the current I1 is determined by the equation I ; further, the secondary voltage E2 and angular frequency ω of said oscillation circuit are measured and the electric power P applied to the weld zone is determined from the equation II (L0, r0W L5, r5 are respectively the self inductances and resistances of a transmission circuit, the primary and secondary sides of the transformer 9, the overall V-shaped part of the pipe to be welded, and the circumferential length part of the pipe, M is mutual inductance, and C is the capacitance of the oscillation circuit).