An image reading device including: a magnetic circuit comprising a magnet disposed on one side of a conveyance plane, the magnetic circuit generating a magnetic field in a direction perpendicular to the conveyance plane; a magnetoresistive effect element chip disposed between the magnetic circuit and the conveyance plane and detecting a strength change of a conveyance direction component of the magnetic field in the magnetic reading area when a medium to be read is conveyed on the conveyance plane; a light guide disposed on a side opposite to the magnetic circuit side with respect to the conveyance plane, and irradiating light onto the magnetic reading area on the conveyance plane; and an image forming optical system disposed on the side opposite to the magnetic circuit side with respect to the conveyance plane.