An illumination device including a light source device includes a light source being constituted by LED(s); a light source side reflective plate to which the light source is fixed; an emission side reflective plate facing the light source side reflective plate; and a fixing means for fixing the both reflective plates, wherein the emission side reflective plate is formed so that a portion thereof facing the light source has the highest optical reflectance and the lowest optical transmittance while the optical reflectance decreases and the optical transmittance increases farther away from the light source, and the distance between the both reflective plates is greatest at a portion where the light source is disposed, while the distance decreases at portions farther away from the portion where the light source is disposed. Thus the device can supply light in a substantially uniform manner from a light irradiation surface.