1. A paper sheet identification device for identifying a stained condition of a paper sheet by detecting a stain on the paper sheet, the paper sheet identification device comprising: an area memory that stores therein in advance information about a target detection area where it is to be detected whether a stain is present on the paper sheet; a pixel number memory that stores therein in advance a number of pixels of an image printed on the target detection area as a reference number of pixels; a paper-sheet-type discrimination unit that discriminates a type of the paper sheet; an image reading unit that reads, from the paper sheet, an image on the target detection area selected based on the type of the paper sheet discriminated by the paper-sheet-type discrimination unit; a pixel-number detection unit that detects a number of pixels in the image read by the image reading unit as a number of read pixels; a calculation unit that calculates a stain value, indicating a stained condition on the target detection area of the paper sheet, by comparing the number of read pixels detected by the pixel-number detection unit and the reference number of pixels stored in the pixel number memory, the reference number of pixels selected based on the type of the paper sheet discriminated by the paper-sheet-type discrimination unit.