When processing using a plasma is performed for an object to be processed, a photodetecting unit sequentially detects emission of two active species having specific wavelengths in a designated period during the processing. On the basis of the emission detection information of the two active species, two approximate expressions of linear functions are obtained in the relationship between the emission intensity and time. The ratio of the two approximate expressions of linear functions and the derivative of the ratio are obtained to form a graph in which the ratio is plotted on the abscissa, the derivative of the ratio is plotted on the ordinate, and the intersection between the average value of the ratio and the average value of the derivative of the ratio is the origin. The ratio and the derivative of the ratio are obtained by using the emission detection information of the two active species during the processing after the designated period. The end point of the plasma processing is determined when the position of the ratio and the derivative of the ratio thus obtained deviates from a predetermined region in the graph.