PURPOSE: To reduce size and weight by interposing a gate-turn-off thyristor between plural primary coils disposed in accordance with the secondary coil of a transformer for resistance welding so that said primary coils can change alternately polarities.

CONSTITUTION: A pair of primary coils 1a, 1b are provided in accordance with a secondary coil 3. The one coil 1a is connected through the one side thereof to a primary terminal T1 and the other side is connected through M and N to the other terminal T2. The one coil 1b is connected at the one side thereof to the terminal T1 through O and P interposed between the same, and the other side is connected through Q and N to the other terminal T2. A gate turn-off thyristor 4 is further arranged between the terminals T1 and L and between the terminals T2 and P so that the coils 1a, 1b change alternately polarities and generate high frequencies.