1. A personal authentication device comprising: an imaging section for imaging a user's eye including the iris thereof; an iris code generating section for generating an iris code from the captured image; a determining section for comparing the generated iris code with a registered iris code stored in advance in a storage section and determining whether the iris code matches or does not match the registered code; a control section for giving an instruction to the imaging section to retry imaging when it is determined there is no match; a guiding section for guiding the user to a position where light reflection in the iris is different during retry-imaging; an average calculation image creating section for extracting an iris image representing the iris from a plurality of captured images if the iris code generated from the image captured by retry-imaging does not match the registered iris code, computing average values for the respective pixels of each of the iris images and creating an average calculation image of the computed average pixel values; and an iris code generation instruction section for instructing the iris code generating section to generate an iris code from the created average calculation image.