PURPOSE: To improve the quality by making the fuzzy theory that the changes of the wall thickness of tube stock and the feeding speed of tube is made to the variable of antecedent part as the method for controlling the output voltage of the welding machine and controlling the welding of the resistance welded tube.

CONSTITUTION: On the welding control of the resistance welded tube that the value of output voltage of the welding machine 3 is calculated with the changes of the wall thickness of tube stock and the feeding speed of the resistance welded tube, and the output voltage of the welding machine 3 is controlled following the above on the welding with the electro-resistance welding the formed tube, the output voltage value of the welding machine 3 is corrected by using the fuzzy theory that the wall thickness change of the resistance welded tube and the change of the feeding speed of the resistance welded tube are made as the variables of the antecedent part. Therefore, the tube stock for the resistance welded tube being formed to the tube is heated feasibly even if it is in an abnormal state, and the quality of the resistance welded tube can be improved.