A system for preventing spoofing to use a service provided on a network. The system holds correlation between a first identifier used by a user of a service provided on a network for first authentication for permitting a user to make connection to the network and a second identifier used for second authentication for permitting the user to use a service, holds correlation between a third identifier assigned to the user permitted to make connection to the network and the first identifier of the user, and checks the correlation between the second identifier and the third identifier contained in the packet for the second authentication.