Upon acquiring first data transmitted from an outside of a predetermined range in a network, an apparatus stores, in a memory, first information including transmission source and destination addresses of the first data. Upon acquiring second data addressed to an inside of the predetermined range and indicating predetermined communication data of service initiation, the apparatus extracts the first information including as the transmission source address a source address of the second data, and stores, in the memory, second information indicating a service initiation and including a destination address of the second data, in association with the first information. When the second information including as the transmission destination address a source address of the second data is stored in the memory and a destination address of the second data coincides with the transmission source address in the first information associated with the second information, the apparatus notifies detection of an attack.