To provide a vibration damping device which can be simply disposed in an engine of a small planing boat which can not have sufficient space.

This vibration damping device is attached to a crank shaft of an engine mounted on a small planing boat to reduce torsional vibration caused by torsional stress acting on the crankshaft. In the vibration damping device, a vibration damping mass 5 is provided, via an elastic member 4, on the outer peripheral surface of a flange member 2 for a coupling mounted on the output end of the crankshaft 1 of the engine E such that it can be elastically moved for the crankshaft 1 in the direction of rotation.