PURPOSE: To contrive effective utilization or the like of space by welding a monocock part to a rear part of right and left body tubes further a reinforcing plate between these monocock part and tubes while arranging a cushion cylinder of a power unit in the inside of the reinforcing plate.

CONSTITUTION: An upper end of a down tube 3 is welded to a head pipe 2, while a front end of right and left body tubes 4 is welded to a lower end of the down tube 3. A rear part of the body tubes 4 is lifted, while a monocock part 5 is welded to this rear part. While a reinforcing plate 6 is welded between a front lower part of the monocock part 5 and a lower part of the body tubes 4 so as to coat them, while a cushion cylinder 17 of a power unit 8 is arranged in the inside of the reinforcing plate 6. While in the monocock part 5, a large pocket 22 is formed in the inside, a seat 23 is openably-closeably mounted to the upper part while a frame cover 24 is arranged in the outside.