as shown in fig1 a breather box bb is provided integrally with the bottom of the air intake box 1 as shown in fig1 the breather box bb is connected to a breather pipe connecting port on the engine e side through a breather pipe bp for example the connecting port is provided in the cylinder head ch of the engine e as shown in fig3 the breather box bb is provided with an oil inlet oi and a tip end of the breather pipe bp is connected to the oil inlet oi the breather box bb is further provided with an oil return port or such that the upper end of the oil return port or is lower than the upper end of the oil inlet oi as shown in fig3 a base end of a breather return pipe br is connected to the oil return port or the breather return pipe br connects the breather box bb to the oil tank to see fig5 as the engine oil reservoir as shown in fig1 it should be appreciated that when the engine e is not of a dry sump type a tip end of the breather return pipe br is connected to the oil reserving portion at the bottom of the crank case of the engine e as shown in fig1 the breather pipe bp is entirely substantially u shaped in other.