A synchronized communication system comprising a first communication device and one or more second communication devices (#1, #2, --- #n) that perform communication in a predetermined communication cycle and respectively comprise first and second PHY sections (110, 210) configured to convert a logic signal into an electrical signal and a link section (140, 240), wherein the second PHY section of each communication device is connected to the first PHY section of another communication device, characterized in that: the first PHY section (110, 210) is configured to connect, respectively, to the second PHY section (110, 210) by a first relay path (160, 260) via a first reception first-in first-out (Rx FIFO) section (130, 230) for receiving data from the first PHY section (110, 210) and a first transmission first-in first-out (Tx FIFO) section (120, 220) for outputting the received data to the second PHY sections (110, 210), whereas the second PHY section (110, 210) is configured to connect, respectively to the first PHY section (110, 210) by a second relay path (160, 260) via a second reception first-in first-out (Rx FIFO) section (130, 230) that receives data from the second PHY section (110, 210) and a second transmission first-in first-out (Tx FIFO) section (120, 220) that outputs the received data to the first PHY sections (110, 210); each of the link sections (140, 240) is connected to the corresponding first and second relay paths (160, 260) and has an in-device reference timer (141, 241) for generating an interrupt signal at a preset timing; the link section of the first communication device is configured to send a synchronization-correction-target specifying frame, a transmission path delay measurement frame and a synchronization frame; each of the first and second relay paths of the one or more second communication devices (#1, #2, --- #n) is inserted with a path selection switch and connected to one end of a return path respectively, the path selection switches being connected to the other end of the return path when the path selection switch disconnects the corresponding first or second relay path, operation of the path selection switches being performed at the same time; wherein the first communication device separately measures transmission path delay times to the one or more second communication devices (#1, #2, ---#n), separately notifies the devices (#1, #2, --- #n) of the corresponding transmission path delay time, and sends a current reference time value in the communication system for each communication cycle; and each of the one or more second communication devices (#1, #2, --- #n) is configured to correct the received current reference time value using the notified corresponding transmission path delay time and set the corrected value in the in-device reference timer (141, 241) thereof. the first PHY section (110, 210) is configured to connect, respectively, to the second PHY section (110, 210) by a first relay path (160, 260) via a first reception first-in first-out (Rx FIFO) section (130, 230) for receiving data from the first PHY section (110, 210) and a first transmission first-in first-out (Tx FIFO) section (120, 220) for outputting the received data to the second PHY sections (110, 210), whereas the second PHY section (110, 210) is configured to connect, respectively to the first PHY section (110, 210) by a second relay path (160, 260) via a second reception first-in first-out (Rx FIFO) section (130, 230) that receives data from the second PHY section (110, 210) and a second transmission first-in first-out (Tx FIFO) section (120, 220) that outputs the received data to the first PHY sections (110, 210); each of the link sections (140, 240) is connected to the corresponding first and second relay paths (160, 260) and has an in-device reference timer (141, 241) for generating an interrupt signal at a preset timing; the link section of the first communication device is configured to send a synchronization-correction-target specifying frame, a transmission path delay measurement frame and a synchronization frame; each of the first and second relay paths of the one or more second communication devices (#1, #2, --- #n) is inserted with a path selection switch and connected to one end of a return path respectively, the path selection switches being connected to the other end of the return path when the path selection switch disconnects the corresponding first or second relay path, operation of the path selection switches being performed at the same time; wherein the first communication device separately measures transmission path delay times to the one or more second communication devices (#1, #2, ---#n), separately notifies the devices (#1, #2, --- #n) of the corresponding transmission path delay time, and sends a current reference time value in the communication system for each communication cycle; and each of the one or more second communication devices (#1, #2, --- #n) is configured to correct the received current reference time value using the notified corresponding transmission path delay time and set the corrected value in the in-device reference timer (141, 241) thereof.