To provide a compact and easy-to-manufacture and -assemble mainframe of a motorcycle.

This mainframe of a motorcycle comprises rear mainframe components such as a steering head pipe 4, a suction box 5 and a pair of right and left side frame members 6 from a front side. The head pipe 4 and at least a front part 5a of the suction box 5 are integrated into a casting. As one example of the structure of the suction box 5, a rear part 5b of the suction box 5 is made of a sheet metal, and jointed to the suction box front part 5a of the casting by welding. For example, an engine mounting boss part 35 and a radiator mounting boss part 34 are integrally formed on the suction box front part 5a. Also, a suction duct connection part 30 and an air cleaner element insertion port are formed.

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