To stably manufacture an electric resistance welded tube having excellent quality.

A metal strip 1 is formed into a circle with a forming machine 2 and an electric resistance welded tube 7 is manufactured by high frequency induction welding. A position of a joining spot 6 is calculated as the position of the center of the gravity of a picture in a high luminance side by binarizing the picture image-picked up with a camera 13 with a picture processor 14. A forming condition of the forming machine 2 is controlled with a forming machine controller 15 so that the position of the joining spot 6 is kept constant. A welding current is detected with a current sensor 10 and controlled so as to be kept constant by controlling an oscillator 8 with an oscillator controller 9.