As the issue of global warming has become more serious, needs for downsizing or weight saving of an engine has been getting stronger, and forces exerted on engine parts, especially force on a crankshaft, have been getting larger and larger. In addition, since a crankshaft is a heavy engine part, needs for saving weight have been getting stronger and stronger. Therefore, determining the mechanism of high stress generation in a crankshaft system is urgently needed.This paper describes the characteristics and mechanism of a severe bending stress caused by the whirl of crankshaft rear end of an inline 6-cylinder medium-duty diesel engine. The authors measured bending stress on the fillets of the crankshaft, and found severe levels of sharp peaks in the stress curves for the crankshaft rear. To figure out why the severe levels of sharp peaks appear, they analyzed and studied the measured data. As a result, they have revealed that the crankshaft rear end whirls, and have determined the mechanism of the crankshaft rear end bending resonance characteristic of the shaft whirling motion.