A plasma processing apparatus having a process chamber in which an object to be processed is subjected to plasma processing includes a light-receiving part for a spectrometer unit, an arithmetic unit, a database, a determination unit and an apparatus controller. The determination unit determines a condition in the processing chamber that an end point of seasoning is reached. The determination of the condition is performed so that one or more differences between one or more output signals derived from a batch of plasma emission data by multivariate analysis and one or more output signals derived from a preceding batch of plasma emission data are found, an average value of the differences in one batch, a difference between a maximum and a minimum of the differences in one batch and a standard deviation of the differences in one batch are determined, and the values are compared with a preset threshold.