A resistance welder controller comprising: a control unit that controls a welding current flowing through an inverter transformer; a current detection unit that detects the welding current flowing through the inverter transformer; characterized by a usage rate calculation unit configured to measure an energizing time and a point time interval of the welding current that has been detected, with means for activating a usage rate measurement mode and calculating a usage rate of the inverter transformer using the energizing time and the point time interval that have been measured, or an average usage rate measurement mode and calculating an average energizing time or an average point time interval for each of the plurality of points or for the fixed time that have been measured and further calculates the usage rate of the inverter transformer using the average energizing time or average point time interval that have been calculated; a storage unit storing an equivalent current curve indicating a relationship between a current value of the welding current and the usage rate of the inverter transformer when the inverter transformer is operated at a rated capacity; and a determination unit configured to determine whether a relationship between the current value of the detected welding current and the calculated usage rate of the inverter transformer, based on the activated measurement mode, exceeds the rated capacity of the inverter transformer based on the equivalent current curve, continues an operation of the control unit when the relationship between the current value of the detected welding current and the calculated usage rate of the inverter transformer does not exceed the rated capacity of the inverter transformer, and stops the operation of the control unit when the relationship between the current value of the detected welding current and the calculated usage rate of the inverter transformer exceeds the rated capacity of the inverter transformer. a control unit that controls a welding current flowing through an inverter transformer; a current detection unit that detects the welding current flowing through the inverter transformer; characterized by a usage rate calculation unit configured to measure an energizing time and a point time interval of the welding current that has been detected, with means for activating a usage rate measurement mode and calculating a usage rate of the inverter transformer using the energizing time and the point time interval that have been measured, or an average usage rate measurement mode and calculating an average energizing time or an average point time interval for each of the plurality of points or for the fixed time that have been measured and further calculates the usage rate of the inverter transformer using the average energizing time or average point time interval that have been calculated; a storage unit storing an equivalent current curve indicating a relationship between a current value of the welding current and the usage rate of the inverter transformer when the inverter transformer is operated at a rated capacity; and a determination unit configured to determine whether a relationship between the current value of the detected welding current and the calculated usage rate of the inverter transformer, based on the activated measurement mode, exceeds the rated capacity of the inverter transformer based on the equivalent current curve, continues an operation of the control unit when the relationship between the current value of the detected welding current and the calculated usage rate of the inverter transformer does not exceed the rated capacity of the inverter transformer, and stops the operation of the control unit when the relationship between the current value of the detected welding current and the calculated usage rate of the inverter transformer exceeds the rated capacity of the inverter transformer.