1. An information searching apparatus comprising: an index information registering device that registers a word extracted from a document in association with the document as index information for document search; a document searching device that searches a document corresponding to information relating to a requested search by referring to the index information registered by said index information registering device; an unknown word extracting device that extracts an unknown word from the document being searched; a document type determining device that determines a type of the document being searched; and a registration permitting/inhibiting determining device that determines whether said index information registering device is permitted or inhibited to register the unknown word extracted by said unknown word extracting device as the index information, according to the type of the document determined by said document type determining device.