There is provided with a monitoring method of monitoring plural apparatuses each having a sensor, including: selecting two or more sample apparatuses out of the plural apparatuses; calculating averages and standard deviations of sensor values detected by the sample apparatuses for each sample apparatus; calculating a confidence interval of the averages and an average of the standard deviations; and calculating an average of sensor values detected by an arbitrary apparatus among the plural apparatuses as an object average and calculating a normal range of the sensor of the arbitrary apparatus from the object average, the confidence interval, and the average of the standard deviations.