To prevent welding defects and to improve yield even when an electric resistance welded tube whose thickness as compared to the outside diameter.

This device is provided with a thicker part forming roll 4 and flat roll 6 for forming thicker parts TA having the end-face thickness W2 which is larger than that of the middle part on both side edges of the sheet bar stock T while making the sheet bar T run, tube forming mechanism for forming the sheet bar stock on which the thick parts TA are formed into a C-shaped cross section by passing through plural forming rolls 10 and welding mechanisms 12, 14, 16 for butting and welding the mutual thicker parts TA after heating both the side edges of the sheet bar stock T which is formed into the C-shaped cross section.