1. An image processing apparatus comprising: an interface configured to receive, from an external apparatus, a first image and a second image, wherein the first image is different from the second image; and a processor configured to: extract a first character string appearing on the first image, and a second character string and a third character string appearing on the second image, cut out each of characters constituting the first character string, in units of a character, cut out each of characters constituting the second character string, in units of a character, and cutting out each of characters constituting the third character string, in units of a character, 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, 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, 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 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.