1. An authentication apparatus comprising: a vein data extracting unit that extracts vein data representing veins, from an image including the veins existing in a finger; an extracting unit that extracts position data representing the position which the contour of the finger has at an intermediate stage of extracting the vein data; and a determining unit that determines a collation candidate to be collated with the vein data, from the similarity between the position data and data associated with vein data to be registered.