An unauthorized access detection device capable of detecting unauthorized accesses which are made through preparation, in real time. When a packet travels on a network, a key data extractor obtains the packet and obtains key data. Next an ongoing scenario detector searches an ongoing scenario storage unit for an ongoing scenario with the key data as search keys. A check unit determines whether the execution of the process indicated by the packet after the ongoing scenario detected by the ongoing scenario detector follows an unauthorized access scenario being stored in an unauthorized access scenario storage unit. Then a report output unit outputs an unauthorized access report depending on the check result of the check unit.