1. A personal authentication apparatus comprising: a light source for irradiating a light to a hand of a user; an imaging unit for creating an image of transmitted light from the hand of the user; a shading element for preventing entering of light other than the transmitted light; a memory unit for storing condition of light to the hand of the user; and a processing unit for extracting a feature of a vein pattern from the image and comparing the feature of the vein pattern with stored feature in the memory unit; wherein the processing unit calculates an irradiation condition which prevents saturation of the image of hand of the user according to a thickness of hand or a thickness of finger of the user and stores the irradiation condition in the memory unit as the irradiation condition of light to the hand of the user.