To provide a method of building a dynamic damper in closed space formed by welding a vehicle body member.

A vehicle body member of hollow beam shape having closed space 12 inside is composed of a body part 10 partially opened, and a closing plate 11 covering this opened part. A mass body 20 is accommodated in the space of the body part 10 before closing it with the closing plate 11, and a bolt 21 is protruded outward from a damper elastic body fitting hole 14 formed in the body part 10, and temporarily fastened to the body part by a nut 26 so as to be conductive. The closing plate 11 is then welded to the body part 10 to form closed space 12. A current is then applied to the body part 10 to perform cation electrostatic coating. After temporary fastening is unfastened, an insulator is fitted to the damper elastic body fitting hole 14, and the bolt 21 is refitted thereto by the nut so as to compose a built-in dynamic damper of a mass body 20 and a damper elastic body constituting the insulator.