1. A control apparatus for an arc welding robot used for both regular welding and crater treatment, the control apparatus comprising: an external interface being operable to obtain at least one actual welding condition during operation of the arc welding robot, the at least one actual welding condition comprising at least one of an actual welding current and an actual welding voltage; an input device being operable to enter a lower allowable range width and an upper allowable range width for a command value for the at least one actual welding condition; a memory being operable to store the lower allowable range width and the upper allowable range width for the command value; a central processing unit being operable to judge if the at least one actual welding condition is within at least one allowable range, the at least one allowable range having a lower value equal to the command value minus the lower allowable range width and an upper value equal to the command value plus the upper allowable range width; and an information noticing device being operable to display operation information of the arc welding robot when the at least one actual welding condition is outside the at least one allowable range.