PROBLEM TO BE SOLVED: To detect the degradation of a fan on the basis of a sensor value by a sensor that can be easily installed to the fan in an addition manner.SOLUTION: An operation monitoring system includes: a plurality of sensor nodes 100 installed in a plurality of frost protection fans 1, respectively； an aggregator 200； and a server 300. Each sensor node 100 includes a vibration sensor for detecting vibration, acquires a sensor value measured by the vibration sensor, calculates, on the basis of the sensor value, a value required for the calculation of the standard deviation of a sensor value group measured in a prescribed time width, and notifies a server 300 of it. The server 300 calculates the standard deviation on the basis of the values reported from the sensor nodes 100, and detects the degradation of the frost protection fan 1 on the basis of the comparison between the standard deviation and a determination threshold.SELECTED DRAWING: Figure 1A
