In this study, we propose a mirror surface finishing method using a spherical polycrystalline diamond (PCD) rotary tool to efficiently finish cemented carbide mold cavities by CNC machining, such as using a machining center. A tool with a ball diameter of ~0.5 mm was trial-manufactured and investigated with regard to chip removal condition, roughness of the finished surface, and changes in internal structure. The results showed that a mirror finish with a surface roughness within 20 nmRa could be achieved, flat-type chips were generated, and with little damage to the structure under the finished surface.