PURPOSE: To reduce noises emitted from the gear chamber of an engine and enhance the gear case supporting strength and tooth bearing of an idle gear.

CONSTITUTION: The front face 7 of an idle gear shaft 5 is joined with a gear case 2 while the rear face 8 is joined with a crank case 1, and the idle gear shaft 5 is pinched fast by the gear case 2 and crank case 1 from the front and rear. Through the idle gear shaft 5, the crank case 1 is held by the central part of the front wall of gear case 2 so that vibrations in the gear chamber 3 are suppressed, so that it is possible not only to reduce noises but also heighten the supporting strength of the gear case 2. The idle gear shaft 5 is two-end supported and its deflection is suppressed, and the teeth bearing of the idle gear 4 is enhanced.