Analysis data constituted by a plurality of parameters is acquired on the basis of detection values obtained in processes for an object to be processed from a detector arranged in a plasma processing apparatus. With respect to parameters of analysis data decided as abnormal data, as a degree of influence on abnormality, a contribution to, e.g., a residual score is calculated (degree-of-influence calculating step). Contributions of the parameters are set at 0 or a value close to 0 in a descending order of contribution of the parameters to sequentially calculate residual scores, and, when the residual scores are not more than a predetermined value, the parameters having the contributions which are set at 0 or a value close to 0 until now as parameters which cause abnormality (cause-of-abnormality deciding step).