To reduce a percussion sound by screwing a plug member, to which an elastic material is fitted, to the percussed-side end section of a shank rod while the size of both members is specified.

One end of a plug-shaped member 3 is screwed to a female tapped hole 2 formed at an end section 1a on the side to be percussed of a shank rod 1 while an elastic body member 5 is inserted into an inserting hole formed at the other end. The diameters of the plug 3 and the elastic member 5 are set in 95-99% and 50-80% of the diameter of the rod 1 respectively, and projecting size from the plug 3 of the elastic member 5 is set in 5-25% of the diameter of the rod 1. The diameters of the plug 3 and the elastic member 5 are set preferably in 98% and 65% of the diameter of the rod 1 respectively, and projecting size from the plug 3 of the elastic member 5 is set preferably in 12% of the diameter of the rod 1. Accordingly, a percussion sound can be reduced without deteriorating substantial drilling capacity, and the percussion- sound reducing mechanism can be executed easily at a low cost because the mechanism may be manufactured only by machining the shank rod as consumables.