A transformer includes a stator core, primary coils, secondary coils, and heat radiating plates each of which projects outward from between interior layers of the primary coil and/or secondary coil. An A.C. arc welder includes the transformer therein, with a movable core which can move to and fro between the primary coils and secondary coils and which has different widths in the movable direction. An A.C. arc welder also includes openings for taking in air at a front portion of a case cover and a fan for exhausting air at a rear portion of the case cover.