To generate large braking force by closing an air bleed hole provided in a low pressure chamber side of a single acting fluid pressure cylinder driving an exhaust brake valve in a fully closed direction, and regulating a discharge amount of air in a low pressure chamber, when an engine speed exceeds a predetermined prescribed value.

In order to operate an exhaust brake, when a foot is detached from an accelerator pedal, compressed air is supplied to each air cylinder, an intake shutter and an exhaust brake valve 7 are closed. Here in the case of an engine in a high speed rotational region, a stepping motor 36 is forwardly rotated by a control device, and through holes 28, 29 of an adjuster 26 ared closed by a movable closing plate 32. In this condition, when the exhaust brake is operated, since air in a low pressure chamber 21c is compressed by a piston 22 in an air cylinder 21, by this compressing resistance, the piston 22 can not be moved to the forefront limit position, a pressure rise in an exhaust manifold is dulled, and an exhaust valve is prevented from opening.