To control to the most a change in the atmospheric pressure of an oxygen sensor (O2 sensor) caused by the action of an exhaust brake and to prevent an erroneous detection of the O2 sensor and to improve the properties of exhaust gas by disposing the O2 sensor in the downstream side of an exhaust shutter.

The exhaust from an engine is discharged to the atmosphere from a muffler via an exhaust manifold and an exhaust duct 10. An o, sensor 13 for detecting oxygen concentration in the exhaust discharged from an exhaust passage and an exhaust brake for increasing the inner resistance of the engine by an exhaust shutter 12 disposed in the exhaust passage and producing a braking force are provided. In particular, the O2 sensor 13 is disposed in the downstream side of the exhaust shutter 12, whereby a change in atmospheric pressure of the O2 sensor 13 caused by the opening/closing of the exhaust shutter 12 is prevented, which can prevent an erroneous detection of the O2 sensor 13 caused by the change in the atmospheric pressure.