An object of the invention is to achieve a small and thin irregular pattern reader comprising: a prism possessing a detection surface on which a subject to be detected having an irregular pattern is put and an incident plane having a first angle of inclination to the detection surface and emitting a light reflected on the detection surface corresponding to an incident light incident upon the incident plane; a first optical system possessing a light source and causing a light from the light source to be incident upon the incident plane of the prism with its optical axis substantially in parallel to the detection surface; and a second optical system for transmitting the emission light emitted from the prism to an image pick-up device.