1. A biometric image output control method performed by a biometric authentication device including an imaging device, a display device and a processor, the method comprising: obtaining a biometric image by imaging a body of a user through the imaging device, determining a separation distance from the imaging device to the body of the user through the imaging device; controlling, by the processor, at least one of: a size of the biometric image, a focal point of the biometric image, and brightness of the biometric image according to the separation distance in order to display a controlled biometric image, wherein the controlling of the size of the biometric image is based on a first function related to the size of the biometric image with respect to the separation distance, wherein the first function is defined such that an obtained result of an absolute value of a ratio of a change of the size versus a change of the separation distance which is taken within a predetermined distance range, being smaller than an obtained result of an absolute value of a ratio of a change of the size versus a change of the separation distance which is taken outside the predetermined distance range, wherein the controlling of the focal point of the biometric image is based on a second function related to the focal point with respect to the separation distance, wherein the second function is defined such that an obtained result of an absolute value of a ratio of a change of the focal point versus a change of the separation distance which is taken within the predetermined distance range, being smaller than an obtained result of an absolute value of a ratio of a change of the focal point versus a change of the separation distance which is taken outside the predetermined distance range, and wherein the controlling of the brightness of the biometric image is based on a third function related to the brightness with respect to the separation distance wherein the third function is defined such that an obtained result of an absolute value of a ratio of a change of the brightness versus a change of the separation distance which is taken within the predetermined distance range, being smaller than an obtained result of an absolute value of a ratio of a change of the brightness versus a change of the separation distance which is taken outside the predetermined distance range matching the controlled biometric image to an authentication image pre-registered in a storage unit.