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 an area where a stain is to be detected on the paper sheet as information about a target detection area; a pixel number memory that stores therein in advance a number of pixels in an image printed on the target detection area as a reference number of pixels used as a reference value when being compared; an image reading unit that, based on the information about the target detection area stored in the area memory, reads an image on the target detection area from the paper sheet; a pixel-number detection unit that detects a number of pixels in the image read by the image reading unit as number of read pixels; and 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.