A synchronized communication system is provided in which even when communication devices are connected in series to a transmission path, devices connected to the communication devices are synchronized so that a communication cycle can be reduced. A second communication device includes a relay path and a return path which do not pass through a storage device, and a path selection switch therefor. A first communication device instructs switching of the return path from the transmission path, measures a transmission path delay time, notifies the second communication device of it, and sends a reference time of the first communication device for each communication cycle. The second communication device corrects its reference time using the transmission path delay time and the reference time. When a new second communication device is found during synchronized communication, the first communication device also performs transmission path delay measurement using the remaining time in the communication cycle.