1. A method for detecting an occurrence of an abnormality in a measurement target, the method comprising: acquiring multiple primary measurement values from the measurement target; calculating multiple reference values so that a reference value is calculated for each primary measurement value using optimal learning; calculating multiple secondary measurement values with the multiple primary measurement values and the corresponding multiple reference values, wherein the multiple secondary measurement values are calculated by subtracting the corresponding multiple reference values from the multiple primary measurement values; calculating a relationship matrix indicating a plurality of mutual relationships among the multiple secondary measurement values; and calculating an anomaly score for each secondary measurement value, which indicates the degree the measurement target is abnormal by comparing a predictive value with the secondary measurement value, wherein the predictive value is calculated based on the relationship matrix and other multiple secondary measurement values.