PURPOSE: To obtain a power source device for welding, which has reduced a power loss and has improved a welding quality, by connecting a primary winding of a reactor to between an AC power source and an electrode, and suppressing a welding current to a prescribed value or below by a current control device connected to a secondary winding, in case of welding by Joule heat such as seam welding, etc.

CONSTITUTION: A primary winding of a reactor to which a secondary winding containing an iron core is connected between an AC power source 6 and welding electrodes 3, 4, and when a welding current exceeds a prescribed value IF by a current control device 8 connected to the secondary winding, an operational amplifier 17 generates an output, an oscillating device 18 executes oscillation, and a base current is intermitted. Therefore, the peak value of a welding current waveform flowing to a welding trasformer 5 is cut to the prescribed value IF, and by controlling the prescribed value IF, the welding current can be controlled to an optional value in a state that a good current waveform is held. According to such constitution, a power loss is reduced, a welding condition of a wide range is obtained, and a welding quality can be improved.