PURPOSE: To attain compatibility between the improvement of strength and light weight by forming a frame of skeleton members, molded by casting or forging, having underfilled parts on one side faces, and cover members rigidly fixed to the skeleton members so as to close the opening parts of the underfilled parts.

CONSTITUTION: A motorcycle frame 10 is provided with a lateral pair of main frame part 13R (illustrated only on the right side) where the base end part of a swing arm for supporting a rear wheel is held and side members 14, 15 are laid among plural parts. The main frame part 13R is formed of a skeleton member 16R and a cover plate 17R. The skeleton member 16R is molded by casting or forging, and recessed parts (underfilled parts) 19R are formed at the center part of the inner side face. The cover plate 17R is formed by punching a plate by press working or the like, and screws are fastened into the threaded holes 22 of the skeleton member 16R through the screw insert holes 28... of the cover plate 17R to cover the skeleton member 16R fixedly with the cover plate 17R.