To reduce the number of components and allow easy tuning by effectively utilizing a mounted object.

In this dynamic damper A, a mass body 2 stored in a U-shaped frame 1 with one face open is prevented from being removed from the frame 1 by inserting a protrusion 7 formed on a first elastic member 3a mounted via a belt 3c into a locking hole 8 formed in the frame 1. The mass body 2 has a second elastic member 3b mounted via the belt 3c together with the first elastic member 3a and attached to the side of the open face of the frame 1. The second elastic member 3b is protruded outward from the mounting face levels of mounting flanges 4 provided on both sides of the opening face of the frame 1. In mounting on the mounted object B, the frame 1 is pushed against the mounted object B in the teeth of the elastic force of the first and second elastic members 3a, 3b and fastened thereto with a bolt 9 and a nut 11.