To prevent sucking-in of dust to an air cleaner, and to improve the maintenance service life by forming an intake air passage for introducing outside air to the air cleaner in a car body frame, and opening the upstream end of the intake air passage outside in an upper part of a seat support part.

An air cleaner 76 is formed by housing a cleaner element 80 in a cleaner case 79, and the cleaner case 79 is composed of a case main part 77 and a cover 78. A purifying chamber 82 continuing with the upstream end of a carburetor 70 and a nonpurifying chamber 83 are formed on both ends of the cleaner element 80 in the cleaner case 79, and the nonpurifying chamber 83 is communicated with the downstream end of an intake air passage 84 formed in a seat support part 19 arranged in a car body frame 10, and the upstream end of the intake air passage 84 is opened outside in an upper part of the seat support part 19 to thereby prevent the air cleaner from sucking dust to the possible minimum level to improve the maintenance service life of the air cleaner.