For the purpose of preventing a situation in which the fiber density looks as if it suddenly decreases in a specific view direction, a method comprises: specifying a region of interest R1 in MR image data collected by a diffusion tensor method; defining regular grid points in the region of interest R1; defining points obtained by randomly moving the grid points as tracking start points S1, S2, . . . ; performing diffusion tensor analysis on each tracking start point S1, S2, . . . in the image data to determine a direction of a principal axis vector; tracking a fiber by repeatedly selecting a neighbor point along the direction of the principal axis vector and performing diffusion tensor analysis on the neighbor point to determine the direction of the principal axis vector; and producing and displaying an image of the tracked fibers as viewed in a desired view direction.