An image processing welding control method comprises detection of isothermal pattern of a weld zone in the welding state through photographing the weld zone by an infrared camera; calculation of the area of the region defined by the isothermal line detected; comparison of the area with a standard area which is obtained in advance; control of welding heat input such that the area will be substantially the same as the standard area whereby a suitable welding heat is input the welding portion; detection of an asymmetric extent of said isothermal pattern; and controlling the torch position such that the asymmetric extent will be within a predetermined value, so that the torch will be strictly aligned with the welding groove center. This method enables two kinds of welding control methods, torch position control and welding heat input control, based on the same image information.