An image reading device comprising: - a magnetic circuit disposed on one side of a conveyance plane, configured to convey a sheet-like medium to be read along a direction of a sheet surface of the medium to be read, and to generate a magnetic field in a direction perpendicular to the conveyance plane; - a magnetoresistive effect element disposed between the magnetic circuit and the conveyance plane and configured to detect an intensity variation in a conveyance direction component of the magnetic field in a magnetic reading area when the medium to be read is conveyed along the conveyance plane; - a light source disposed on a side opposite to the magnetic circuit side with respect to the conveyance plane and configured to irradiate the magnetic reading area of the conveyance plane with light; and - an imaging optical system and a light receiver disposed on the side opposite to the magnetic circuit side with respect to the conveyance plane, wherein the imaging optical system forms an image of an optical reading area, which includes at least a portion in the conveyance direction of the magnetic reading area in the conveyance plane, on the light receiver, characterized in that the magnetic circuit, the magnetoresistive effect element, the imaging optical system, and the light receiver each extend in a main scanning direction that is parallel to the conveyance plane and orthogonal to the conveyance direction, and the magnetic reading area is a portion of the optical reading area along the main scanning direction, and wherein the length in the main scanning direction of the magnetic reading area is shorter than the length in the main scanning direction of the optical reading area. - a magnetic circuit disposed on one side of a conveyance plane, configured to convey a sheet-like medium to be read along a direction of a sheet surface of the medium to be read, and to generate a magnetic field in a direction perpendicular to the conveyance plane; - a magnetoresistive effect element disposed between the magnetic circuit and the conveyance plane and configured to detect an intensity variation in a conveyance direction component of the magnetic field in a magnetic reading area when the medium to be read is conveyed along the conveyance plane; - a light source disposed on a side opposite to the magnetic circuit side with respect to the conveyance plane and configured to irradiate the magnetic reading area of the conveyance plane with light; and - an imaging optical system and a light receiver disposed on the side opposite to the magnetic circuit side with respect to the conveyance plane, wherein the imaging optical system forms an image of an optical reading area, which includes at least a portion in the conveyance direction of the magnetic reading area in the conveyance plane, on the light receiver, characterized in that the magnetic circuit, the magnetoresistive effect element, the imaging optical system, and the light receiver each extend in a main scanning direction that is parallel to the conveyance plane and orthogonal to the conveyance direction, and the magnetic reading area is a portion of the optical reading area along the main scanning direction, and wherein the length in the main scanning direction of the magnetic reading area is shorter than the length in the main scanning direction of the optical reading area.