A biometric authentication apparatus includes a biometric information acquiring unit which captures an image of a first body part of a user carrying biometric information to generate a biometric image representing the biometric information, and a processing unit configured to determine a position of a second body part of the user on a position detection image to determine the user's position relative to a reference position, determine a rotation angle corresponding to the user's position based on a relationship between the user's position relative to the reference position and the rotation angle, rotate registered biometric information of a registered user or the biometric information on the biometric image according to the rotation angle so as to make the direction of the registered biometric information substantially parallel to the direction of the biometric information, and compare the registered biometric information and the biometric information to each other.