1. A plasma processing unit for conducting a predetermined etching process to an object to be processed, the processing unit comprising: a plurality of detectors; and a controlling unit that estimates or monitors a processing state based on a plurality of detected data from the plurality of detectors, wherein the controlling unit includes: a first storage unit that stores the plurality of detected data; a second storage unit that stores constant response variables respectively set for two states before and after the processing state changes, the response variables being different from each other, the two states being a state while the etching process is in progress and a state after completion of the etching process; a third storage unit that stores a multivariate analysis program, according to which a multiple regression analysis using a PLS method about the response variables is conducted in order to produce a model expression, predictor variables of the multiple regression analysis being the plurality of detected data; a first computing unit that executes the multivariate analysis program to produce the model expression; a fourth storage unit that stores the produced model expression; and a second computing unit that applies the plurality of detected data to the model expression in order to estimate or monitor the processing state, the plurality of detected data being detected when the predetermined etching process is conducted to the object to be processed, wherein the plasma processing unit is adapted to estimate or monitor a change of the processing state from the ongoing (in-progress) state of the etching process to the completion of the etching process based on response variables obtained by applying the plurality of detected data to the model expression.