5. The character string recognition apparatus according to claim 1 ; wherein the means for checking the character string validity calculates character string positions in a direction perpendicular to the direction of the character string for each of the first segments, and checks the presence of unusual data in the character strings according to first-order differences of positions of the character string, and then judges that the character strings are predetermined character strings if no unusual data is detected in the character strings.
5. The character string recognition apparatus according to claim 1 ; wherein the means for checking the character string validity calculates character string positions in a direction perpendicular to the direction of the character string for each of the first segments, and checks the presence of unusual data in the character strings according to first-order differences of positions of the character string, and then judges that the character strings are predetermined character strings if no unusual data is detected in the character strings. 8. The character string recognition apparatus according to claim 5 ; wherein the character string positions are character string center positions.