A fault diagnostic system (1) of the present invention includes a first fault diagnostic device (3) and a second fault diagnostic device (5). The first fault diagnostic device (3) acquires the voltage value of a battery (2) in accordance with a synchronization signal, and sends the voltage value and the synchronization signal. The second fault diagnostic device (5) acquires the voltage value of an inverter (4) in accordance with the received synchronization signal, matches the acquisition timings of the voltage value of the battery (2) and the voltage value of the inverter (4) on the basis of the received synchronization signal, and performs a fault diagnosis by comparing these voltage values.