A diesel engine oil dilution managing device is configured to act as an oil dilution detecting device to determine if a premature combustion occurred before fuel injection by an injector based on an engine parameter that correlates with an occurrence of the premature combustion, such as an amount of time required for the crankshaft to rotate per unit crank angle or a rotational speed change rate of the crankshaft. Moreover, the diesel engine oil dilution managing device can have an engine output control section that is configured to suppress the production of output power by the engine when it is determined that the premature combustion occurred before the fuel injection by, for example, driving a throttle valve in a close direction.