An image-reading apparatus includes a light source unit that includes, a light source section on which a plurality of light-emitting elements that emit light toward a medium whose image is read by an image-capturing unit are arranged in a main-scanning direction, and a diffusion unit that diffuses therein the light emitted from the light-emitting elements, wherein a rib provided in a frame of the image-reading apparatus, the rib being capable of being fitted with a cutout portion, the light source section includes a connector provided at one end of the main-scanning direction and the cutout portion provided at the other end of the main-scanning direction, the diffusion unit has a shape through which the light source section is inserted, and a size of the diffusion unit in the main-scanning direction is equal to or shorter than a distance between the connector and the cutout portion.