The present invention is a monitoring method of monitoring a change of a processing state of an object to be processed when a predetermined process is conducted to the object to be processed by using a processing unit. The method includes: a step of respectively setting constant response variables for two states before and after a processing state changes, the response variables being different from each other; and a step of conducting a multiple regression analysis about the response variables in order to produce a model expression, predictor variables of the multiple regression analysis being a plurality of detected data from a plurality of detectors provided in the processing unit. Then, the method includes: a step of actually obtaining a plurality of detected data from the plurality of detectors when the predetermined process is conducted to the object to be processed; and a step of estimating or monitoring a processing state by applying the obtained plurality of detected data to the model expression.