Abstract: Operation information about a component of an information system is acquired by a CMDB (configuration management database). An investigation information DB stores assumption narrowing information including a plurality of inquiry items to be issued to a user to narrow a cause of a fault of an information system to a specific assumption, and assumption verification information including information necessary for verification of an assumption for each of a plurality of assumptions included in the assumption narrowing information. An inquiry item optimizing function unit refers to the assumption verification information stored in the investigation information DB and operation information stored in the CMDB, generates priority assignment information necessary in assigning a priority to the inquiry item, and assigns the priority to the inquiry item included in the assumption narrowing information on the basis of the priority assignment information.