The present invention relates to a method of synchronous control for a synchronous control system provided with a master device and at least one slave device connected via a communications network. The master device transmits first time data to the master device and the at least one slave device upon detection of generation of a synchronization signal, the first time data indicating a time at which the synchronization signal is generated. The master device transmits second time data to the at least one slave device upon reception of the first time data, the second time data indicating a time at which the first time data is received. Each of the at least one slave device calculates, upon reception of the first time data and the second time data, an estimated generation time at which the synchronization signal is estimated to be generated by subtracting time elapsed between the time indicated by the first time data and the time indicated by the second time data from time at which the first time data was received. Each of the at least one slave device controls synchronization with the master device based on the estimated generation time.