1. A fingerprint minutia input device comprising a main device body which comprises: an image output controller which outputs/displays a fingerprint image containing fingerprint ridgelines and a pointer for designating a coordinate position of a minutia of the fingerprint image on a display screen; and a minutia input setting unit which sets/displays a minutia display mark for the minutia on the fingerprint image designated by the pointer according to an external command, wherein: the main device body comprises a gray-scale value adjustment unit which sets gray-scale intensity of the entire fingerprint image where the pointer is located to be used for gray-scale adjustment within a display gray-scale range constituted with a gray-scale upper limit value and a gray-scale lower limit value according to an external command; and the screen output controller includes a gray-scale width update function which updates/outputs output display of the fingerprint image with the display gray-scale width set anew by the gray-scale value adjustment unit when setting/displaying the minutia display mark, and a gray-scale width fixing/holding function which fixes and holds the set display gray-scale width as the display gray-scale width for displaying/inputting the minutia when there is no new setting information of the display gray-scale width from the gray-scale value adjustment unit.