Monitoring system, particle beam therapy system, and method of repairing plant

A monitoring and diagnosis system reduces a load at the time of collecting operation state information of a plant apparatus and is inexpensive. A plant includes a plant apparatus, a local control device that controls the plant apparatus on the basis of operation state information of the plant apparatus from a sensor, and a high-order control system that is connected to the local control device and controls the operation of the entire plant. A monitoring subsystem performs failure diagnosis in accordance with a failure diagnosis algorithm which is set in advance, on the basis of the operation state information acquired from the local control device. A monitoring and diagnosis system collects results of the failure diagnosis performed by the monitoring subsystem and pieces of operation state information of the plant apparatus which are consolidated by the monitoring subsystem.