A reverse rotation preventing device for a Diesel engine which can stop, when the engine exhibits a stopping tendency, operation of the engine to prevent reverse rotation of the engine with certainty and can automatically release a stopping interlock after stopping of the engine. The device comprises an engine stopping mechanism, an engine speed detecting mechanism, a comparing mechanism for comparing a detection value developed from the engine speed detecting mechanism with a reference value which is set in advance as a low engine speed with which stopping of the engine may be resulted, a judging mechanism for judging in accordance with the detection value whether or not the engine is in a deceleration tendency, an outputting mechanism for outputting an engine stopping signal to the engine stopping mechanism when the detection value is lower than the reference value and the engine is in a deceleration tendency, and a timer mechanism for interrupting the outputting of the engine stopping signal after lapse of a predetermined interval of time.