The present invention provides an illumination device including a cylindrical light source and a curved mirror for reflecting light radiated from the cylindrical light source, wherein the curved mirror has a light reflection surface which has a shape of a portion of an elliptic curve having a first focal point and a second focal point on a reference axis of the curved surface, in a cross-sectional surface perpendicular to the axial direction of the light source, and the cylindrical light source is disposed on the reference axis at a position between the first focal point and the second focal point, a light irradiation apparatus including the illumination device, and a method for producing a photoreaction product sheet using the irradiation apparatus.