To provide an unauthorized use determination server and method, capable of easily attaining monitoring of unauthorized use.

The unauthorized use determination server 1 includes a log-out history storage means 12 which stores, in response to receipt of a log-out request of a predetermined service containing a user ID, a log-out history in association with the user ID; a log-out history complement means 14 which determines, in response to receipt of a log-in request to the predetermined service, whether the latest log-out history is present by referring to a log-in/log-out history DB 21, takes an access history stored just before in association with the user ID related to the log-in request as the latest log-out history in reference to an access history DB 22 when the latest log-out history is absent, and complements it; an unauthorized use determination means 15 which checks for unauthorized use of the predetermined service based on the stored log-out history and the complemented access history; and a determination result output means 16 which outputs the determination result.

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