An image reading device is provided. The image reading device suppresses uneven intensity of light emitted on a subject, by disposing a diffusion plate between the subject and a plurality of luminescence portions, and an image forming device that includes this image reading device. The image reading device includes an illumination portion that irradiates light onto a subject. The illumination portion includes: a plurality of LED's disposed at predetermined intervals in a main scanning direction; and an elliptical diffuser plate that is disposed between the subject and the plurality of LED's and diffuses light emitted by the plurality of LED's in a higher diffusion rate in the main scanning direction than in a sub-scanning direction.