A compact authentication device that prevents user from feeling pressure and is strong against external light, when capturing an image of a finger blood vessel pattern with transmitted light. The device includes a guidance part for determining the finger position, a light source disposed on at least one side of the guidance part to emit light to be transmitted though the finger, an image capture part for capturing the transmitted light, a shading unit for limiting an irradiation region of the light, a finger thickness measuring unit, a unit for controlling a light amount of the light source based on a result of the measurement, a unit for recording registered image patterns of the finger, a unit for collating a captured image pattern from the image capture part with the registered patterns, and a unit for controlling different processing according to the collation result.