Factors which are adversely affecting a specific quality of products are extracted promptly and easily for common use by analyzing the causal relation between product quality data and quality affecting data. The apparatus comprises a memory unit 2 for saving the product quality data and the data which may adversely affect the product quality detected during a processing operation such as a diffusion process at a semiconductor plant, and a multistage multivariate analysis unit 4 for analyzing the relationship between the quality data as object variables and the quality affecting data as explanation variable saved in the memory unit 2. The analysis is conducted at multiple stages to reduce the number of the explanation variables to a fixed number so that possible abnormal items (explanation variables) are automatically screened at each stage, until the abnormal factors are extracted at the last stage.