A communication device: stores flow condition information for identifying a flow and flow counter information that indicates, for each flow, an input flow volume of a flow inputted to the communication device, and an output flow volume of a flow outputted by the communication device; identifies a flow to which data inputted to the communication device belongs, with reference to the flow condition information; updates the input flow volume of the flow in the flow counter information; identifies a flow to which data outputted by the communication device belongs, with reference to the flow condition information; updates the output flow volume of the flow in the flow counter information; and identifies a flow in which a communication anomaly has occurred on the basis of results of a comparison process for comparing the input flow volume to the output flow volume with reference to the flow counter information.