PURPOSE: To obtain many kinds of gages of good accuracy by drafting the shapes of the gages for the grooves of pipe end faces in the case of welding a pipe and a branch pipe with an electronic computer.

CONSTITUTION: Input data including the diameters of a main pipe 1, and a branch pipe 2 to be welded to the pipe 1, their installation position, the shape of a groove, etc. are written on a data sheet 3 and are inputted to an electronic computer 4. Next, the computer 4 calculates the groove shapes of the end faces of the branch pipe 2 at their every division angle with the ablove-mentioned data and the operating means storing these beforehand and inputs the same to a plotter 5. In the plotter 5, the groove shape of a full size is drafted on paper 6. The groove shape drafted on the paper 6 is cut off, and is affixed to a sheet 9. Many gage pieces for the welding groove are cut off from the sheet 9.