An illumination apparatus to illuminate a sample surface with excellent illumination efficiency and a reflective characteristics measuring apparatus using the illumination apparatus. The illumination apparatus includes a plane light source positioned on a normal at a center of the sample surface and a mirror having an internal reflective surface positioned between the plane light source and the sample surface. The internal reflective surface has a circular or polygonal shape in a section perpendicular to the normal and the circular or polygonal shape substantially corresponds to an imaginary circle centered on the normal and having a radius equal to half a distance between the plane light source and the sample surface. In place of the mirror, a plurality of reflective faces may be positioned.