In the voltage sensor diagnostic device, a monitoring device diagnoses a normality/abnormality of a voltage sensor that detects a circuit voltage in a system circuit of a vehicle-mounted motor control system. The monitoring device includes a main multiplier circuit, a subordinate multiplier circuit and an abnormality diagnosis circuit. The main multiplier circuit receives a circuit voltage as an input monitoring voltage and outputs a gain-non-inverted signal as a detected value of the circuit voltage. The subordinate multiplier circuit receives the circuit voltage as the input monitoring voltage and outputs a gain-inverted amplified with respect to the gain-non-inverted output signal from the main multiplier circuit as the detected value. The abnormality diagnosis circuit diagnoses abnormalities in the main multiplier circuit and the subordinate multiplier circuit, which are voltage sensors, based on the gain-non-inverted output signal from the main multiplier circuit and the gain-inverted output signal from the subordinate multiplier circuit.