In order to inspect the inner failure of gear units without dismantling through analysis of wear particles in lubricant, the following two types of gear failures are intentionally caused and examined. One type is abnormal wear acceleration caused by contamination in lubricant. The other is seizure between pinion and gear. Gear oil is sampled periodically during the test. Analyzing the wear particles in the sampled oil by ferrography, relationship between typical failures in gear unit and features of wear particles generated in oil is revealed.