A verification server is configured to communicate with a usage target system via a first communication channel and an information terminal device via a second communication channel. The verification server includes: a unit for registering personal information of a user for using a usage target system; a unit for receiving, via the second communication channel, system identification information of the usage target system and a restriction code from an information terminal device owned by a use; a unit for generating an internal system password for the usage target system; a unit for receiving, via the first communication channel, a plurality of characters from the usage target system, the plurality of characters being inputted by a user into the usage target system; and a unit for determining whether the plurality of characters are legitimate based on the internal system password and the system identification information.