A process of forming a rugged pattern on a golf ball surface, the process comprising the steps of: 1. assuming a large number of circles on a surface of a phantom sphere, wherein the number of the circles is equal to or greater than 280 but equal to or less than 450, and the large number of circles are assumed such that each circle does not intersect another circle adjacent to the circle; 2. assuming a large number of generating points based on positions of the large number of circles, wherein the center of each circle is assumed as a generating point; 3. assuming a large number of Voronoi regions on the surface of the phantom sphere by a Voronoi tessellation based on the large number of generating points, wherein the step 3 comprises the steps of: 3.1 assuming a large number of minute cells on the surface of the phantom sphere; 3.2 selecting a generating point that is closest to each cell; 3.3 assuming, for each generating point, a set of cells for which said each generating point is a closest generating point; and 3.4 setting each set as the Voronoi region; and smoothing a contour of each Voronoi region by moving averaging, wherein a plurality of points having the coordinates obtained by the moving average are connected to each other by a spline curve; and 4. assigning a dimple and a land to the surface of the phantom sphere based on contours of the large number of Voronoi regions, wherein a loop is obtained by the spline curve, wherein a land is assigned onto the loop or to the outside of the loop and a dimple is assigned to the inside of the loop or onto the loop; and wherein the occupation ratio of the dimples is equal to or greater than 85% and equal to or less than 98%. 1. assuming a large number of circles on a surface of a phantom sphere, wherein the number of the circles is equal to or greater than 280 but equal to or less than 450, and the large number of circles are assumed such that each circle does not intersect another circle adjacent to the circle; 2. assuming a large number of generating points based on positions of the large number of circles, wherein the center of each circle is assumed as a generating point; 3. assuming a large number of Voronoi regions on the surface of the phantom sphere by a Voronoi tessellation based on the large number of generating points, wherein the step 3 comprises the steps of: 3.1 assuming a large number of minute cells on the surface of the phantom sphere; 3.2 selecting a generating point that is closest to each cell; 3.3 assuming, for each generating point, a set of cells for which said each generating point is a closest generating point; and 3.4 setting each set as the Voronoi region; and smoothing a contour of each Voronoi region by moving averaging, wherein a plurality of points having the coordinates obtained by the moving average are connected to each other by a spline curve; and 3.1 assuming a large number of minute cells on the surface of the phantom sphere; 3.2 selecting a generating point that is closest to each cell; 3.3 assuming, for each generating point, a set of cells for which said each generating point is a closest generating point; and 3.4 setting each set as the Voronoi region; and smoothing a contour of each Voronoi region by moving averaging, wherein a plurality of points having the coordinates obtained by the moving average are connected to each other by a spline curve; and 4. assigning a dimple and a land to the surface of the phantom sphere based on contours of the large number of Voronoi regions, wherein a loop is obtained by the spline curve, wherein a land is assigned onto the loop or to the outside of the loop and a dimple is assigned to the inside of the loop or onto the loop; and wherein the occupation ratio of the dimples is equal to or greater than 85% and equal to or less than 98%.