To provide a device and method for discriminating sheets capable of preventing misidentification occurring due to adhesion of dust inside an imaging section and preventing use in abnormal state by detecting abnormalities inside the imaging section due to adhesion of dust and notifying the abnormalities outside.

When the sheets are inserted into a sheet inserting section 3, the sheets are carried by a sheet carrying section 4. The imaging section 7 picks up pictures inside the imaging section 7 before carrying of the sheets into the imaging section 7 and acquires image data without the sheets. When the sheets are carried into the imaging section 7, the imaging section 7 picks up the sheets and acquires image data with the sheets. A dust detection section 8 measures the position and area of the image indicating the dust in the image data without the sheets. When there is an image at the same position with an identical area indicating the dust in the image data with the sheets, the image is discriminated as image indicating the dust adhered inside the imaging section 7 in the image data with the sheets. When the identified dust is out of specification, notification of abnormality of the imaging section 7 is sent to a control section 2.