1. A plasma processing method for monitoring information on a plasma processing based on a model or for updating an operating condition of the plasma processing based on a change in the information on the plasma processing caused by the model, when performing the plasma processing on an object by setting the operating condition of a processing apparatus and generating a plasma in an airtight processing vessel provided in the processing apparatus, wherein the model is made by performing a multivariate analysis based on measurement data measured by one or more measuring devices installed at the processing apparatus, the method comprising the steps of: obtaining first measurement data by using the measuring devices while performing the plasma processing by setting a first operating condition; formulating a first model based on the first measurement data by the multivariate analysis; obtaining second measurement data by using the measuring devices while performing the plasma processing by setting a second operating condition; formulating a second model based on the second measurement data by the multivariate analysis; obtaining third measurement data by using the measuring devices while performing the plasma processing by setting a third operating condition; obtaining weight factors by setting the third measurement data as weighted measurement data wherein the weighted measurement data is obtained by multiplying each of the first and the second measurement data by one of the weight factors to produce first and second weighted data and summing the thus produced first and the second weighted data; and formulating a third model for the third operating condition by multiplying each of the first and the second model by said one of the weight factors to produce first and second weighted models, and summing the thus produced first and the second weighted models.