1. A character string recognition method for recognizing a character string comprising the steps of: imaging a character string on a medium to obtain image data; calculating, in a first step, a first projection data of the image data in a direction of the character string while shifting in a perpendicular direction which is perpendicular to a direction of the character string; and detecting, in a second step, a position of the character string in the perpendicular direction on a basis of the first projection data which are obtained through the first step; wherein, in the first step, the image data are divided into a plurality of segments in the direction of the character string and projection in the segment is calculated.