A linear light source device illuminates a document properly, and an image reading device includes such a light source device. The light source device includes a light source and a light-guiding member. The light-guiding member introduces the light emitted from the light source from one end portion in x-direction, cause the light to travel in x-direction, and emits the light from a light exit portion at one end in z-direction, which is orthogonal to x-direction. The light-guiding member includes, at the other end in z-direction, a light-reflecting portion that reflects light to the light exit portion. One end face of the light-guiding member in y-direction, which is orthogonal to x- and z-directions, is inclined so as to become closer to the other end face in y-direction with decreasing distance from the light exit portion in z-direction. This allows light to easily travel to the light-reflecting portion.