According to an aspect of an embodiment, a method for controlling an apparatus for transferring data from a plurality of first devices to a second device via a network, the data being transferred by using a packet, the method comprises the steps of: extracting encryption information identifying method of encryption conveyed by a packet and destination information identifying destination of the packet transmitted from one of the first devices; counting the number of kinds of the destination information extracted from packets associated with the same encryption information, respectively; and determining an unauthorized communication when the number of kinds of the encryption information is less than a predetermined value.