A system monitoring apparatus has: a determination unit for determining whether the relatedness indicating a relationship that is true for first time-series data of multiple sets measured during a first time period is true for second time-series data of the multiple sets measured during a second time period; an irregularity degree calculation unit for calculating the degree of irregularity indicating the extent to which the second time-series data is irregular; a first extent calculation unit for calculating a first extent indicating the extent to which the irregularity is at a specified value when the second time-series data are normal or irregular; a second extent calculation unit for calculating a second extent indicating the extent to which the second time-series data of the multiple sets are related; and a state calculation unit for finding whether the second time-series data are normal or irregular, based on the first and second extents.