An exhaust throttle valve protected from disturbance due to stones scattered during operation of a vehicle. Front side exhaust pipes and rear side exhaust pipes are connected to a catalyst chamber disposed below a crankcase. The catalyst chamber and an exhaust chamber disposed to the rear thereof are connected to each other via a rear exhaust pipe. The diameter of the rear exhaust pipe is smaller than the lateral width of each of the catalyst chamber and the exhaust chamber, so that a recessed space is externally formed so as to recede inside of the vehicle. An exhaust throttle valve is received in the space. The exhaust throttle valve is partially exposed to the outside of the rear exhaust pipe and a throttle valve is received in the rear exhaust pipe to throttle-control the passage-sectional area of the rear exhaust pipe according to the rotation of the engine.