1. A search apparatus for searching for a keyword from a character recognition result using an index table, the character recognition result being obtained as a result of character recognition of characters in an original document, wherein the index table includes an index character string; a position of a portion, in the character recognition result, which matches the index character string; and a credibility which is defined for each character included in the index character string and indicates a probability of the character existing in a portion, in the original document, which corresponds to a portion, in the character recognition result, which matches the character, the search apparatus comprising: a position specifying section for determining whether or not the keyword matches a portion in the character recognition result based on the index character string and the position of the portion in the character recognition result which matches the index character string included in the index table; and when a match is determined, for specifying the position of the portion in the character recognition result which matches the keyword; and a calculation section for calculating a keyword credibility which indicates a probability of the keyword existing at a position, in the original document, which corresponds to the position of the portion in the character recognition result which matches the keyword, based on the credibility included in the index table.