This vehicle-mounted air conditioner control device (1) comprises: a PTC heater (H) that is included in a high-voltage system circuit (HV) and generates heat by the power supplied from a high-voltage battery (B2); and a microcomputer (10) that is included in the high-voltage system circuit (HV) and controls the power supply from the high-voltage battery (B2) to the PTC heater (H). This vehicle-mounted air conditioner control device (1) further comprises: a water temperature sensor (151, 152) that is included in a low-voltage system circuit (LV) and outputs a voltage signal corresponding to the temperature of hot water; and a V/f conversion unit (161, 162) that is included in the low-voltage system circuit (LV) and converts the voltage signal output from the water temperature sensor (151, 152) into a frequency signal. This vehicle-mounted air conditioner control device (1) further comprises a digital isolator (171, 172) that transmits the frequency signal to the microcomputer (10) while maintaining electrical insulation between the V/f conversion unit (161, 162) and the microcomputer (10).