The effects of the combination of hardness and surface roughness on the surface durability of medium-hardness gears (HB300∼HB400) are examined by using a disk machine, when the pinion with a smooth surface is harder than the gear with a rough surface in order to improve the load-carrying capacity. In increasing the initial hardness of the disk with a smooth surface (1μm Rmax) in comparison with the hardness of the disk with a rough surface (10μm Rmax), the pitting life and wear resistance of the disk with a smooth surface are improved. But the pitting limit of the disk with a rough surface is not improved. These facts indicate that it is necessary for the pinion with a smooth surface to be harder than the gear with a rough surface in order to improve the surface durability of the pinion rather than the gear, when only the pinion tooth surface is smoothed between a pair of gears.