1. A method of discriminating longitudinal melanonychia, including: a first step of assuming a digital color image of nail apparatus melanoma as three-dimensional vectors each composed of RGB parameter values of each pixel and finding a latitudinal variable and a longitudinal variable for each of the three-dimensional vectors in an RGB space; a second step of finding a discrimination index according to a distribution of points defined by the latitudinal and longitudinal variables found in the first step; and a third step of classifying the discrimination index found in the second step according to a threshold and discriminating whether the nail apparatus melanoma is malignant or benign.