1. An image processing apparatus comprising: an extraction unit configured to extract a first character string appearing on a first image, and a second character string and a third character string which appear on a second image; a cut-out unit configured to cut out each of characters constituting the first character string, in units of a character, to cut out each of characters constituting the second character string, in units of a character, and to cut out each of characters constituting the third character string, in units of a character; a calculation unit configured to calculate a first similarity degree group which is composed of similarity degrees between the respective characters constituting the first character string and respective candidate characters in a candidate character group, to calculate a second similarity degree group which is composed of similarity degrees between the respective characters constituting the second character string and the respective candidate characters in the candidate character group, and to calculate a third similarity degree group which is composed of similarity degrees between the respective characters constituting the third character string and the respective candidate characters in the candidate character group; and a determination unit configured to determine which of the second character string and the third character string corresponds to a first subject which is identical to the first character string, by comparing the first similarity degree group with the second similarity degree group and the third similarity degree group.