PURPOSE: To decrease remarkably the weight of a welding transformer by installing the welding transformer to a welding gun, and feeding power to a spot welding point by a coaxial electric conductor, in a high frequency spot welding machine provided with a frequency converter.

CONSTITUTION: A welding gun 9 is moved to a welding point position of a pair of alloy plates 17 to be welded, and by operating a pressure device, the welding point of the pair of alloy plates to be welded 17 is pressed by inserting and holding it between electrode chips 15, 16 and a back bar electrode 18a. By turning on an invertor device 11, a high frequency power is supplid to a welding transformer 13 installed to the welding gun 9 through a coaxial primary cable 12 distributed along an arm of a welding robot, and a high frequency power of a low voltage and a large current of its secondary side is supplied to the series spot welding electrodes 15, 16 through a coaxial cable 14. The supplied welding current heats and deposits the joint surface of the lower part of both the electrodes 15, 16 through the back bar electrode 18a.