A pitting fatigue of gear teethes is one of the major failure mode of gears used for construction machineries.A two-cylinder type rolling contact fatigue testing is widely used for investigating the fatigue characteristics.It was reported that a tribofilm is formed on the cylinder surface during the test and the film highly affects fatigue life.As shown in Fig.1,it was also reported that the friction force is changed by the formation of the tribofilm.