Usually, the conditions of finished tooth surfaces are different between a pair of pinion and gear of the medium-hardness gears (the hardened and tempered gears), because their sizes, materials and hardness are often different each other. The authors investigate the influence of roughness combination of a pair of gears on their surface durability using a disk machine with rolling and sliding. The first report deals with the case of the same hardness combination, and the results are summarized as follows : (i) The pairs of disks with the combination of the surface roughness 10 μm and 1 μm Rmax have the pitting limits of about 0.26 HB kgf/mm2, while the pitting limit of disk pairs with 10 μm Rmax is about 0.1 HB kgf/mm2. (ii) However, the disk with smooth surface (1 μm Rmax) is worn out considerably, and further the pits of the lower speed disk with smooth surface occur earlier than those of the higher speed one with rough surface. (iii) When a pinion with smooth surface mates with a gear with rough surface, the above facts indicate that the pitting and wear resistances of pinion should be made higher than that of gear.