1. A character string detection device for detecting a character string including at least one character in an image, the character string detection device comprising: a clustering unit that defines at least one cluster including at least a fixed number of pixel groups having similar colors based on color information included in each pixel configuring the image; a clipping unit that partitions the image into a region of the pixel groups belonging to the cluster defined by the clustering unit and a region of other pixel groups, and generating a clipped image excluding the region of the other pixel groups from the image; and a character string detection unit that detects each character in a detection target character string to be detected so as to detect the detection target character string in the clipped image generated by the clipping unit, wherein the character string detection unit decides, upon detection of one character included in the detection target character string in the image a character to be detected subsequently out of undetected characters closest to the already detected character in the aligned characters in the detection target character string, the character string detection unit limits a detection target region to regions on a right of and below the already detected character if the already detected character is an n-th character in the aligned characters in the detection target character string and the character to be detected subsequently is an (n+1)-th character, and the character string detection unit limits the detection target region to regions on a left of and above the already detected character if the character to be detected subsequently is an (n−1)-th character.