In a digital color image of longitudinal melanonychia, RGB parameter values of each pixel are assumed as three components to form a three-dimensional vector. Latitudinal and longitudinal variables are obtained for each three-dimensional vector in an RGB space. The latitudinal and longitudinal variables are used to define a distribution of points, and from the distribution, a discrimination index is found. The discrimination index is classified according to a threshold, to discriminate whether the longitudinal melanonychia is malignant or benign. The distribution of points is displayed to realize visualization of the malignancy of the longitudinal melanonychia.