An inverter power supply comprising: a power source configured to apply a reverse voltage increasing gradually to a rectifier element (172, 174), wherein the rectifier element (172, 174) is a diode (172, 174) of a diode stack being provided in an inverter transformer which is included in a resistance welder; an isolation amplifier functioning as a current detection unit configured to detect a current value of a current flowing through the rectifier element (172, 174) when the reverse voltage is applied; and characterised in that the inverter power supply further comprises: a control unit configured to decide that the rectifier element (172, 174) has deteriorated when the detected current value is larger than a determination current value for determining the deterioration of the rectifier element (172, 174), and configured to decide that the rectifier element has not deteriorated when the detected current value is smaller than the determination current value for determining the deterioration of the rectifier element (172, 174); a shifting unit configured to shift between a normal mode in which a welding current is supplied to the resistance welder and a measurement mode in which deterioration of the rectifier element (172, 174) is determined, wherein when the shifting unit has been shifted to the normal mode, the power source and the isolation amplifier stop functioning, and when the shifting unit is shifted to the measurement mode, the power source and the isolation amplifier start functioning, and wherein the diode stack is further provided with a cooling water passage that cools the diode (172, 174), and the shifting unit is configured such that cooling water is circulated through the cooling water passage when the shifting unit has been shifted to the normal mode, and the cooling water is removed from the cooling water passage when the shifting unit has been shifted to the measurement mode. a power source configured to apply a reverse voltage increasing gradually to a rectifier element (172, 174), wherein the rectifier element (172, 174) is a diode (172, 174) of a diode stack being provided in an inverter transformer which is included in a resistance welder; an isolation amplifier functioning as a current detection unit configured to detect a current value of a current flowing through the rectifier element (172, 174) when the reverse voltage is applied; and characterised in that the inverter power supply further comprises: a control unit configured to decide that the rectifier element (172, 174) has deteriorated when the detected current value is larger than a determination current value for determining the deterioration of the rectifier element (172, 174), and configured to decide that the rectifier element has not deteriorated when the detected current value is smaller than the determination current value for determining the deterioration of the rectifier element (172, 174); a shifting unit configured to shift between a normal mode in which a welding current is supplied to the resistance welder and a measurement mode in which deterioration of the rectifier element (172, 174) is determined, wherein when the shifting unit has been shifted to the normal mode, the power source and the isolation amplifier stop functioning, and when the shifting unit is shifted to the measurement mode, the power source and the isolation amplifier start functioning, and wherein the diode stack is further provided with a cooling water passage that cools the diode (172, 174), and the shifting unit is configured such that cooling water is circulated through the cooling water passage when the shifting unit has been shifted to the normal mode, and the cooling water is removed from the cooling water passage when the shifting unit has been shifted to the measurement mode. a control unit configured to decide that the rectifier element (172, 174) has deteriorated when the detected current value is larger than a determination current value for determining the deterioration of the rectifier element (172, 174), and configured to decide that the rectifier element has not deteriorated when the detected current value is smaller than the determination current value for determining the deterioration of the rectifier element (172, 174); a shifting unit configured to shift between a normal mode in which a welding current is supplied to the resistance welder and a measurement mode in which deterioration of the rectifier element (172, 174) is determined, wherein when the shifting unit has been shifted to the normal mode, the power source and the isolation amplifier stop functioning, and when the shifting unit is shifted to the measurement mode, the power source and the isolation amplifier start functioning, and wherein the diode stack is further provided with a cooling water passage that cools the diode (172, 174), and the shifting unit is configured such that cooling water is circulated through the cooling water passage when the shifting unit has been shifted to the normal mode, and the cooling water is removed from the cooling water passage when the shifting unit has been shifted to the measurement mode.