A biometric authentication apparatus comprising: a storage unit arranged to store first shape data representing a shape of a hand vein pattern of a registered user in a state in which fingers of the hand take a first posture corresponding to a fingers-closed state, a first index representing a degree of openness of the registered user in the first posture, second shape data representing a shape of the hand vein pattern of the registered user in a state in which the fingers of the hand take a second posture corresponding to a fingers-open state, and a second index representing a degree of openness of the registered user in the second posture, the degree of openness in the first posture differing from the degree of openness in the second posture with respect to, for at least one pair of two adjacent fingers, an angle between the two adjacent fingers; a biometric information acquisition unit arranged to generate a biometric image representing a hand vein pattern of a user; a biometric region extraction unit arranged to extract a biometric region capturing the hand of the user from the biometric image; an invariable region extraction unit arranged to extract, from the biometric region, a first invariable region in which a part of the hand vein pattern of the user is not deformed regardless of finger postures of the hand, and set up a reference coordinate system based on the first invariable region; a posture specification unit arranged to calculate a third index representing a degree of openness of the user in a third posture of fingers of the hand of the user captured in the biometric image from the biometric region in accordance with the reference coordinate system, wherein the degree of openness represents an angle of each finger with respect to an axis of the reference coordinate system or a distance between roots of two adjacent fingers; a biometric information extraction unit arranged to generate third shape data representing a shape of the hand vein pattern of the user captured in the biometric image based on the biometric image; a correction unit arranged to obtain corrected shape data by correcting the first shape data or the second shape data to cancel a shape difference of the hand vein patterns due to a difference between the third posture and the first posture or the second posture according to a ratio of a difference between the third index and the first index or the second index to a difference between the first index and the second index for each finger; and a matching unit arranged to calculate a degree of similarity between the third shape data and the corrected shape data, and authenticate the user when the degree of similarity is equal or greater than a threshold. a storage unit arranged to store first shape data representing a shape of a hand vein pattern of a registered user in a state in which fingers of the hand take a first posture corresponding to a fingers-closed state, a first index representing a degree of openness of the registered user in the first posture, second shape data representing a shape of the hand vein pattern of the registered user in a state in which the fingers of the hand take a second posture corresponding to a fingers-open state, and a second index representing a degree of openness of the registered user in the second posture, the degree of openness in the first posture differing from the degree of openness in the second posture with respect to, for at least one pair of two adjacent fingers, an angle between the two adjacent fingers; a biometric information acquisition unit arranged to generate a biometric image representing a hand vein pattern of a user; a biometric region extraction unit arranged to extract a biometric region capturing the hand of the user from the biometric image; an invariable region extraction unit arranged to extract, from the biometric region, a first invariable region in which a part of the hand vein pattern of the user is not deformed regardless of finger postures of the hand, and set up a reference coordinate system based on the first invariable region; a posture specification unit arranged to calculate a third index representing a degree of openness of the user in a third posture of fingers of the hand of the user captured in the biometric image from the biometric region in accordance with the reference coordinate system, wherein the degree of openness represents an angle of each finger with respect to an axis of the reference coordinate system or a distance between roots of two adjacent fingers; a biometric information extraction unit arranged to generate third shape data representing a shape of the hand vein pattern of the user captured in the biometric image based on the biometric image; a correction unit arranged to obtain corrected shape data by correcting the first shape data or the second shape data to cancel a shape difference of the hand vein patterns due to a difference between the third posture and the first posture or the second posture according to a ratio of a difference between the third index and the first index or the second index to a difference between the first index and the second index for each finger; and a matching unit arranged to calculate a degree of similarity between the third shape data and the corrected shape data, and authenticate the user when the degree of similarity is equal or greater than a threshold.