Currently, the carbon condition, which is supposed to be one of major causes of the sludge in diesel engine oil, is not well reflected to evaluation method for oil filter performance. Therefore we did not have an appropriate evaluation method for the By-Pass Element before this study. Our goal is to develop a laboratory evaluation method of the By-Pass-Element. It has been achieved through the study of the following processes. 1. Analysis of the engine oil sludge on a vehicle monitored. 2. Development of an unique and similar contaminant with the sludge for the evaluation. 3. Development of the evaluation method of the By-Pass-Element by using the contaminant.