1. A character string extraction method in which a character frame is set on a paper sheet image obtained by capturing a paper sheet on which a character string is printed or typed, and an image of the character string is extracted therefrom, the character string extraction method comprising: setting a plurality of character portions for the character string, each character portion is a region including at least one individual character of the character string; setting a rim portion that is a surrounding region adjacent to and around the character portion; setting a character frame that is a combination of the character portion and the rim portion; setting a character string frame that includes all the character frames for the character string; calculating feature values of each image in the character portion and the rim portion for each character frame; calculating a character string frame evaluation value based on the feature values for the character string frame; moving a position of the character string frame on the paper sheet image; and extracting an image in the character portion by using the character string frame at a position at which the character string frame evaluation value reaches a maximum.