For a long time, the tooth surface strength of gears has been investigated under various conditions. At first research concerning tooth surface strength focused mainly on gears of lower hardness. Because the practical usage of gears with high hardness has recently increased considerably, a more fundamental research on tooth surface strength of gears with high hardness is required to increase the load carrying capacity of gears. Recently, many basic studies on the tooth surface strength of gears were carried out in order to improve tooth surface strength. But further research on tooth surface strength is still needed. In view of this present state of research, the authors developed a project to carry out preparatory experiments mainly on the following topics: (1) tooth surface strength using various lubricating oils, (2) tooth surface strength under various effective depths of case-hardening of tooth surfaces, (3) tooth surface strength using test gears with shot peening treatment, (4) tooth surface strength under various tooth surface treatments, and (5) tooth surface strength as a function of tooth hardness.