PURPOSE: To cause the turbine of an air compressor to consume the energy of a crankshaft, and obtain an auxiliary brake without necessity of cooling and troublesome adjustment by connecting the turbine to the crankshaft of an internal combustion engine and providing an open/close valve at the delivery port of the air compressor.

CONSTITUTION: The crankshaft 2 of an internal combustion engine 1 is connected with the turbine 4 of an air compressor 3 as an auxiliary for a main brake. Also, the output shaft 5 of the crankshaft 2 is connected to a wheel power transmission shaft 6 and provided with an increasing gear 7. Furthermore, the input shaft 8 of the turbine 4 is molded monolithically with the output shaft 9 of an increasing gear 7. On the other hand, the air compressor 3 is so constituted as to have a delivery port 13 in the radial outward direction of an impeller 10 in a casing 11, and a valve 15 at said delivery port 13 for adjusting the opening area thereof. And whenever necessary, a switch 21 is turned ON and a driving means 16 is operated, thereby actuating the valve 15 and opening the delivery port 13. According to the aforesaid system, braking power working upon the crankshaft 2 is augmented.