A damper which can be easily fitted to the engine E of a personal watercraft W having little space, said damper damping the torsional vibration caused by the torsional stress acting on the crankshaft 1 of the engine E mounted in a personal watercraft W. A flange 2, which is a component of the coupling D that couples the crankshaft 1 and the propeller shaft 11 of a propulsion pump P, is fixed to the end of the crankshaft 1 which is adjacent to the pump P. The damper includes a damping mass fixed via an elastomeric member 4 to the periphery of a flange 2 in such a manner that the mass 5 can elastically move with respect to the crankshaft 1 in the direction of rotation.