In a cluster system: a heartbeat transmission unit in a service provision node transmits to a monitoring node a heartbeat packet which contains interval information indicating an interval greater than normal intervals at which normal heartbeat packets are transmitted, when an execution unit in the service provision node receives a stop command. Thereafter, when software realizing the execution unit is updated and the execution unit is restarted, the heartbeat transmission unit periodically transmits the normal heartbeat packets to the monitoring node at the normal intervals, where the normal heartbeat packets contain interval information indicating the normal intervals. A heartbeat monitoring unit in the monitoring node determines that the execution unit is in operation, while the monitoring node receives heartbeat packets at intervals indicated by the interval information stored in the monitoring node and updated with received interval information.