There is provided an information searching apparatus that can carry out proper searches with few missed search hits and few erroneous search hits. A word extracted from a document is registered in association with the document as index information for document search. A document corresponding to information relating to a requested search is searched by referring to the registered index information. An unknown word is extracted from the document being searched. A type of the document being searched is determined. It is determined whether the index information registering device is permitted or inhibited to register the unknown word extracted as the index information, according to the determined type of the document determined.