Abstract: A secondary conductor supervising device of a resistance welder control system for supervising short-circuit, disconnection, and/or deterioration of a secondary conductor by means of a current flowing through a resistance welder, which comprises a voltage detecting circuit for detecting a voltage from the secondary conductor, inclusive of a secondary cable, of the resistance welder, a current detecting circuit for detecting a current flowing through the resistance welder, a divider for dividing the output of the voltage detecting circuit by the output of the current detecting circuit, an average computer for averaging the output of the divider over a part or the whole of a welding period, a supervisory level setting unit for setting a resistance value such as of the secondary conductor, and a comparator for comparing the output of the setting unit with the output of the average computer based on the welding period.