1. A personal identification system comprising: a light source that shines a light on an imaging region of a living body; an imaging unit that detects a transmitted light from the imaging region for capturing an image of the living body; and an image processing unit that extracts blood vessel patterns of the living body from the image, transformed by said imaging unit, for comparison with blood vessel patterns registered in advance, wherein an amount of light from said light source is optimized according to image information on the captured image.