1. A character string recognition apparatus, for recognizing a character string through processing image data obtained by imaging the character string placed on a medium, comprising: means for setting a first character string search area on the medium; means for preparing segments for making up a first plurality of segments by segmenting the character string search area in a direction of the character string; means for calculating projection for calculating a projection of the image data in direction of the character string for each of the first segments; means for calculating a number of character string lines for each of the first segments according to the projection of the image data; means for determining the number of character string lines in the character string search area as a whole according to the number of character string lines in each of the first segments; and means for checking the character string validity for verifying whether or not the character string is a predetermined character string, according to the number of character string lines in the character string search area as a whole.