A control device for propulsion units is adapted to synchronize the engine rotational speeds of a plurality of propulsion units arranged in a row on a vessel and operatively and electrically connected to two control levers that are positioned adjacent to each other. The control device synchronizes the engine rotational speeds by correcting the throttle opening of a target propulsion unit or units based on a deviation between the engine rotational speed of a reference propulsion unit and the engine rotational speed of the target propulsion unit. Upon cancellation of synchronization, the throttle opening correction is reduced stepwise from its corrected throttle opening to its natural throttle opening based on the position of the corresponding control lever. As such, large fluctuations in engine speeds are avoided upon cancellation of synchronization.