1. A welding controlling method using image processing comprising the steps of: detecting a isothermal pattern having at least two isothermal lines, different in temperature, of a weld zone in the welding state; detecting the existence of displacement of the centroid of said each isothermal line in a perpendicular direction to a welding direction; controlling a relative position of a welding torch to a material to be welded such that said displacement is substantially zero is there is displacement, thereby to restore an optimum relative position of said torch to the material; calculating the area of the region defined by one of said detected isothermal lines; comparing said area with a preset reference area; and controlling welding heat input such that these areas are substantially the same.