A fault diagnostic system for diagnosing a fault in a first device and a second device connected by a communication line, comprising: a first fault diagnostic device comprising: a first state quantity acquisition unit configured to acquire a first state quantity indicating a state of the first device at a time of update of a synchronization signal; and a first communication unit configured to send the first state quantity and the synchronization signal through the communication line; and a second fault diagnostic device comprising: a second communication unit configured to receive the first state quantity and the synchronization signal through the communication line; a second state quantity acquisition unit configured to acquire a second state quantity indicating a state of the second device at the time of update of the synchronization signal received by the second communication unit; and a comparative diagnostic unit configured to match acquisition timings of the first state quantity and the second state quantity with reference to the synchronization signal received by the second communication unit, and perform a fault diagnosis by comparing the first state quantity and the second state quantity of the same acquisition timing. a first fault diagnostic device comprising: a first state quantity acquisition unit configured to acquire a first state quantity indicating a state of the first device at a time of update of a synchronization signal; and a first communication unit configured to send the first state quantity and the synchronization signal through the communication line; and a first state quantity acquisition unit configured to acquire a first state quantity indicating a state of the first device at a time of update of a synchronization signal; and a first communication unit configured to send the first state quantity and the synchronization signal through the communication line; and a second fault diagnostic device comprising: a second communication unit configured to receive the first state quantity and the synchronization signal through the communication line; a second state quantity acquisition unit configured to acquire a second state quantity indicating a state of the second device at the time of update of the synchronization signal received by the second communication unit; and a comparative diagnostic unit configured to match acquisition timings of the first state quantity and the second state quantity with reference to the synchronization signal received by the second communication unit, and perform a fault diagnosis by comparing the first state quantity and the second state quantity of the same acquisition timing. a second communication unit configured to receive the first state quantity and the synchronization signal through the communication line; a second state quantity acquisition unit configured to acquire a second state quantity indicating a state of the second device at the time of update of the synchronization signal received by the second communication unit; and a comparative diagnostic unit configured to match acquisition timings of the first state quantity and the second state quantity with reference to the synchronization signal received by the second communication unit, and perform a fault diagnosis by comparing the first state quantity and the second state quantity of the same acquisition timing.