The insulation deterioration monitoring apparatus comprises a computer on which software is installed. The software causes the computer to execute at least first to fourth processes. In the first process, during stopping of at least one inverter unit, the computer selects one inverter unit out of the stopped inverter units and causes the inverter corresponding to the selected inverter unit to generate a direct current voltage. In the second process, the computer measures the current value of the ground current while the corresponding motor driving inverter generates the direct current voltage. In the third process, the computer records the measured current value of the ground current for each inverter unit in association with the selected inverter unit. Then, in the fourth process, based on the measured current value of the ground current recorded for each inverter unit, the computer analyzes the tendency of insulation deterioration of each inverter unit.