An anomaly detecting device includes a flow collector that collects an amount of flow communication traffic in each of two or more networks in an in-vehicle network system that including the two or more networks, the amount of flow communication traffic being information obtained by tallying an amount of communication traffic of one or more frames classified according to a predetermined rule that is based on header information of a network protocol; and an anomaly detector that calculates, based on the amount of flow communication traffic, an observed ratio indicating a ratio of respective amounts of communication traffic in the two or more networks and determines whether the two or more networks are anomalous based on the observed ratio calculated and a normal ratio indicating a ratio of respective amounts of communication traffic in the two or more networks in a normal state.