An information processing system, having one or more information processing apparatuses, includes a data input unit configured to take as input first data being multidimensional; a dimension reduction unit configured to generate, based on the first data, second data representing a characteristic of the first data, the second data having a prescribed number of dimensions fewer than a number of dimensions of the first data; and a distinguishing unit configured to distinguish whether the first data is normal data or abnormal data by a semi-supervised anomaly detection, based on the first data and the second data.