Monitoring system

A monitoring system includes monitoring devices and a controller configured to wirelessly communicate with the monitoring devices to acquire monitoring information of a monitored device. In a non-operating state of the monitored device, the monitoring devices establish communication connections between the monitoring devices and periodically perform communication through the communication connections. One monitoring device acts as a communication master and another one monitoring device acts as a communication slave in each of the communication connections. Each of the monitoring devices acts as a communication master for another of the monitoring devices and also as a communication slave for another of the monitoring devices. In the non-operating state of the monitored device, the controller disconnects a communication connection between the controller and each of the monitoring devices, or performs communication with at least one of the monitoring devices less frequently than the communication between the monitoring devices.