A multi-hop relay system including a transmission node and a relay node, wherein the transmission node and the relay node are configured to communicate with each other using a flooding method, wherein the transmission node is configured to generate and transmit a synchronization packet used to synchronize a plurality of relay nodes at a predetermined cycle, and the relay node is configured to: perform first intermittent reception for waiting for the synchronization packet in a first period at every first cycle corresponding to the predetermined cycle; perform second intermittent reception for waiting for the synchronization packet in a second period longer than a length of the predetermined cycle at every second cycle longer than the first cycle, wherein the second intermittent reception is performed when the synchronization packet is not received by the first intermittent reception; determine whether the received synchronization packet satisfies a predetermined condition when the synchronization packet is received by the second intermittent reception; execute synchronization processing, stop the second intermittent reception and perform the first intermittent reception when it is determined that the predetermined condition is satisfied; and execute synchronization recovery processing when it is determined that the predetermined condition is not satisfied, wherein the synchronization recovery processing includes at least one of resetting processing of the first period, reactivation processing of the relay node, reactivation processing of a program executed by the relay node, updating processing of the program of the relay node, and updating processing of firmware of the relay node. the transmission node is configured to generate and transmit a synchronization packet used to synchronize a plurality of relay nodes at a predetermined cycle, and the relay node is configured to: perform first intermittent reception for waiting for the synchronization packet in a first period at every first cycle corresponding to the predetermined cycle; perform second intermittent reception for waiting for the synchronization packet in a second period longer than a length of the predetermined cycle at every second cycle longer than the first cycle, wherein the second intermittent reception is performed when the synchronization packet is not received by the first intermittent reception; determine whether the received synchronization packet satisfies a predetermined condition when the synchronization packet is received by the second intermittent reception; execute synchronization processing, stop the second intermittent reception and perform the first intermittent reception when it is determined that the predetermined condition is satisfied; and execute synchronization recovery processing when it is determined that the predetermined condition is not satisfied, wherein the synchronization recovery processing includes at least one of resetting processing of the first period, reactivation processing of the relay node, reactivation processing of a program executed by the relay node, updating processing of the program of the relay node, and updating processing of firmware of the relay node. perform first intermittent reception for waiting for the synchronization packet in a first period at every first cycle corresponding to the predetermined cycle; perform second intermittent reception for waiting for the synchronization packet in a second period longer than a length of the predetermined cycle at every second cycle longer than the first cycle, wherein the second intermittent reception is performed when the synchronization packet is not received by the first intermittent reception; determine whether the received synchronization packet satisfies a predetermined condition when the synchronization packet is received by the second intermittent reception; execute synchronization processing, stop the second intermittent reception and perform the first intermittent reception when it is determined that the predetermined condition is satisfied; and execute synchronization recovery processing when it is determined that the predetermined condition is not satisfied, wherein the synchronization recovery processing includes at least one of resetting processing of the first period, reactivation processing of the relay node, reactivation processing of a program executed by the relay node, updating processing of the program of the relay node, and updating processing of firmware of the relay node.