An illumination apparatus adapted to illuminate a line-like illumination zone on a road, the illumination zone having a first direction, which is a longitudinal direction, and a second direction, which is a lateral direction, crossing the first direction, the illumination apparatus comprising: a light source adapted to emit a coherent light beam; and a diffraction optical device adapted to diffract the coherent light beam incident from the light source , wherein the diffraction optical device is adapted to diffract the incident coherent light beam so that a width of the illumination zone in the second direction gradually becomes wider along the first direction of the illumination zone from a nearer side to the diffraction optical device , and the illumination zone viewed from a normal direction of the illumination zone becomes a trapezoid which has the shortest length in the second direction at one end of the illumination zone in the first direction closer to the diffraction optical device and the longest length in the second direction at another end of the illumination zone in the first direction farthest from the diffraction optical device , wherein a diffraction angle of the coherent light beam diffracted by the diffraction optical device in the second direction of the illumination zone is constant in the arr entire zone of the illumination zone in the first direction, thereby forming the trapezoidal shaped illumination zone , characterized in that the diffraction optical device is adapted to diffract the coherent light beam such that the trapezoidal shaped illumination zone is visually perceived by the eyes of a viewer, present in the vicinity of the diffraction optical device , as a line-like rectangle extending along the first direction. a light source adapted to emit a coherent light beam; and a diffraction optical device adapted to diffract the coherent light beam incident from the light source , wherein the diffraction optical device is adapted to diffract the incident coherent light beam so that a width of the illumination zone in the second direction gradually becomes wider along the first direction of the illumination zone from a nearer side to the diffraction optical device , and the illumination zone viewed from a normal direction of the illumination zone becomes a trapezoid which has the shortest length in the second direction at one end of the illumination zone in the first direction closer to the diffraction optical device and the longest length in the second direction at another end of the illumination zone in the first direction farthest from the diffraction optical device , wherein a diffraction angle of the coherent light beam diffracted by the diffraction optical device in the second direction of the illumination zone is constant in the arr entire zone of the illumination zone in the first direction, thereby forming the trapezoidal shaped illumination zone , characterized in that the diffraction optical device is adapted to diffract the coherent light beam such that the trapezoidal shaped illumination zone is visually perceived by the eyes of a viewer, present in the vicinity of the diffraction optical device , as a line-like rectangle extending along the first direction. <EOS>