1. A vein authentication device, comprising: an imaging part for continuously imaging a portion of venation present inside a finger by near-infrared light scattered inside the finger by irradiating a finger surface with the near-infrared light while continuously imaging a portion of fingerprints present on the finger surface or inside the finger by reflected light reflected on the finger surface or inside the finger; a motion vector detection part for detecting a motion vector of the fingerprints based on a plurality of images picking up a portion of the fingerprints; an image synthesis part for synthesizing images picking up a portion of the fingerprints based on the motion vector of the fingerprints to generate an image of the fingerprints and synthesizing images picking up a portion of the venation based on the motion vector of the fingerprints to generate an image of the venation; a vein pattern extraction part for extracting a vein pattern from the image of the venation; and an authentication part for performing authentication processing based on the extracted vein pattern, wherein the imaging part includes: a lens array having a plurality of light receiving lenses disposed like an array and is divided into an area to receive the reflected light and an area to receive transmitted light after passing through the venation; a near-infrared light irradiation source provided at an edge on a side of the area to receive the reflected light of the lens array to irradiate the finger surface with near-infrared light; and an imaging device for generating an image picking up a portion of the fingerprints based on the reflected light and generating an image picking up a portion of the venation based on the transmitted light.