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.
2. The character string recognition apparatus according to claim 1 ; wherein the means for determining the number of character string lines creates a frequency distribution on the number of first segments showing a predetermined number of character string lines according to data of the number of character string lines in each of the first segments, and determines the number of character string lines in the character string search area as a whole, with the number of character string lines having a maximum frequency.