1. A control device for a marine engine for controlling the marine engine having a crankshaft to which a rotor of a motor-generator is directly connected, comprising: crank angle position detection means for detecting each specific crank angle position that appears every time the crankshaft of said engine rotates in an instantaneous speed detection section set to be sufficiently narrower than a crank angle section corresponding to each stroke of a combustion cycle of said engine; instantaneous rotational speed detection means for detecting, as an instantaneous rotational speed of said engine, a rotational speed of said engine detected from a time period between when the last specific crank angle position is detected and when this specific crank angle position is detected, every time said crank angle position detection means detects each specific crank angle position, and storing data including information on the detected instantaneous rotational speed; assist necessity determination means for determining whether a drive force needs to be externally applied to the crankshaft of said engine to assist said engine, from the degree of reduction in the instantaneous rotational speed detected by said instantaneous rotational speed detection means; and motor-generator drive means for driving said motor-generator so as to apply the drive force from said motor-generator to the engine when said assist necessity determination means determines that said engine needs to be assisted.