With respect to two or more NC data points among a group of dots together representing a tool locus, a curvature radius of the tool locus is determined. Then, the NC data point and its succession with other points is displayed in a display color determined according to the magnitude of the curvature radius. Alternatively, a movement direction of each minute segment relative to a specific axis selected from the three (XYZ) axes is determined as either positive, negative, or zero, and the minute segment or an end point thereof is display with the display property determined according to the movement direction. Still alternatively, after a specific axis is selected from the three (XYZ) axes, a region relative to the selected region is divided along the selected axis, for predetermined dividing width, first beginning with a predetermined reference point, and the resultant divided regions are given predetermined display properties. Then, a tool locus is displayed in the three-axial (XYZ) system according to the display properties given to the respective divided regions.