Biodiesel fuel can be used in diesel engines with no major modification, but there are some issues derived from the properties of the fuel. Engine oil dilution is a major issue caused by lower volatility and low oxidation stability in biodiesel fuel. The purpose of this study was to clarify the influence of oil dilution by biodiesel fuel on oxidative degradation characteristics, including the acid value (AV), carbon residue (CR), and kinematic viscosity of diesel engine lubricant oil. Degradation assessment was carried out on lubricant oil during operation of a small diesel engine generator, as well as an oxidative acceleration test using a mixture of biodiesel and lubricant oil. It was found that the kinematic viscosity decreased to 23% from its initial value, the dilution rate increased almost linearly, amounting to 2.8 mass-% after 102 hours of engine operation, and deterioration was greater in JASO DH-1 grade lubricant oil mixed with biodiesel than in JASO DH-2.