In an open/close control device 40 installed in a diesel engine 1, when a rotational speed N of the diesel engine 1 is equal to or higher than an intermediate rotational speed and a fuel injection amount is equal to or less than an idling injection amount Fi, a control portion 33 of a valve opening and closing controlling means 30 determines that the diesel engine is in such a state that engine brake is to be actuated, and controls to close both a bypass valve 24 and an EGR valve 52. Accordingly, since there is no fear that exhaust gas returns from an inlet line side of an exhaust turbine 22 to an outlet line side of a compressor 21, the engine brake can be effectively actuated during running at high speeds and running on a downward slope.