a welding line can be rapidly extracted using three - dimensional cad data constituted of a large number of line sections containing welding line candidates . the method of choosing a welding line includes : a step of specifying one face from two faces of a member where a welding line is to be formed ; a step of specifying the other face from the two faces of the number where a welding line is to be formed ; a step of extracting an edge line ; a step of selecting an edge line zone where welding is possible ; a step of creating welding line information by unifying edge lines where welding is possible ; and a step of correcting welding line information in accordance with the groove shape .