A palm vein authentication apparatus comprising: an illumination device comprising a light source provided on a surface of a substrate and including a plurality of light emitting elements configured to emit light, the light source including a plurality of light emitting elements having a wavelength band that may be regarded as a single color; wherein there is provided a diffraction optical element having a plurality of diffraction gratings and configured to diffract the light from the light source into illumination light that illuminates an illuminated region of an illuminated target, wherein the plurality of light emitting elements are grouped into a plurality of light emitting element groups and each of the plurality of diffraction gratings diffracts the light emitted from each of the plurality of light emitting element groups to a corresponding one of a plurality of irradiating regions (51-1 - 51-4) that are combined to form the illuminated region; wherein the plurality of irradiating regions have identical shapes and identical sizes that are smaller than a shape and a size of the illuminated region, and a shape and a size of the plurality of irradiating regions in an overlapped state are identical to the shape and the size of the illuminated region; and wherein the illuminated region is larger than an area occupied by the diffraction optical element and the light source in a top view of the illumination device viewed from above the surface of the substrate; an imaging optical system configured to capture a biometric image of the illuminated target including the illuminated region; a detector configured to detect a brightness of the biometric image; a judging unit configured to judge whether the brightness of the biometric image satisfies a reference; a correction unit configured to correct a light intensity distribution of each of the plurality of irradiating regions (31-1 - 31-8, 51-1 - 51-4, 510) by controlling a current value supplied to each of the plurality of light emitting elements of the light source so that the brightness of the biometric image satisfies the reference; and a collating unit configured to collate the biometric image satisfying the reference with biometric templates, to perform a biometric authentication; wherein the correction unit is configured to control the current value supplied to each of the plurality of light emitting elements of the light source until a uniformity of the light intensity distribution of each of the plurality of irradiating regions falls within a predetermined range; and wherein the correction unit corrects the light intensity distribution of each of the plurality of irradiating regions by independently controlling the current value supplied to each of the plurality of light emitting elements in each of the plurality of light emitting element groups. an illumination device comprising a light source provided on a surface of a substrate and including a plurality of light emitting elements configured to emit light, the light source including a plurality of light emitting elements having a wavelength band that may be regarded as a single color; wherein there is provided a diffraction optical element having a plurality of diffraction gratings and configured to diffract the light from the light source into illumination light that illuminates an illuminated region of an illuminated target, wherein the plurality of light emitting elements are grouped into a plurality of light emitting element groups and each of the plurality of diffraction gratings diffracts the light emitted from each of the plurality of light emitting element groups to a corresponding one of a plurality of irradiating regions (51-1 - 51-4) that are combined to form the illuminated region; wherein the plurality of irradiating regions have identical shapes and identical sizes that are smaller than a shape and a size of the illuminated region, and a shape and a size of the plurality of irradiating regions in an overlapped state are identical to the shape and the size of the illuminated region; and wherein the illuminated region is larger than an area occupied by the diffraction optical element and the light source in a top view of the illumination device viewed from above the surface of the substrate; an imaging optical system configured to capture a biometric image of the illuminated target including the illuminated region; a detector configured to detect a brightness of the biometric image; a judging unit configured to judge whether the brightness of the biometric image satisfies a reference; a correction unit configured to correct a light intensity distribution of each of the plurality of irradiating regions (31-1 - 31-8, 51-1 - 51-4, 510) by controlling a current value supplied to each of the plurality of light emitting elements of the light source so that the brightness of the biometric image satisfies the reference; and a collating unit configured to collate the biometric image satisfying the reference with biometric templates, to perform a biometric authentication; wherein the correction unit is configured to control the current value supplied to each of the plurality of light emitting elements of the light source until a uniformity of the light intensity distribution of each of the plurality of irradiating regions falls within a predetermined range; and wherein the correction unit corrects the light intensity distribution of each of the plurality of irradiating regions by independently controlling the current value supplied to each of the plurality of light emitting elements in each of the plurality of light emitting element groups.