1. A text search apparatus comprising: a storage that stores index information associating each of characters or character strings contained in a document to be searched including plural categories with each of appearance positions of the characters or the character strings within the document; an acquirer that acquires a search keyword; a forward matching searcher that executes a forward matching search using the index information to retrieve a character or a character string beginning with the search keyword from characters or character strings that belong to a category to be subject to the forward matching search in the document; a partial match searcher that executes a partial match search using the index information to retrieve a character or a character string containing the search keyword from the document; and an outputter that outputs a result of the search by the forward matching searcher and a result of the search by the partial match searcher.