Measurement of the uncertainty used for quality control typically involves a plurality of factors. When the uncertainty exceeds a clinical permissible value, time is required for a medical technologist to investigate and to determine the factor causing the uncertainty. It is thus beneficial to automatically investigate factors in complicated uncertainty, particularly from the view point of reagents and samples which are subject to quality change and that are prone to affect the measurement quality. Quality control samples having a plurality of concentration levels are measured to calculate the average, coefficient of variation, standard deviation, and other numerical values. When quality control samples having n (n≧2) different concentration levels are measured, variation patterns determine the factor causing the uncertainty, the factor being specific to each of 3n different combinations of variation patterns.