An open/close controller (40) installed in a diesel engine (1), wherein when the 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, the control part (33) of an open/close valve opening control means (30) determines that the diesel engine is in such a state that engine brake must be applied thereto, and controls to close both a bypass valve (24) and an EGR valve (52). Accordingly, since such a possibility that exhaust gas returns from the inlet passage side of an exhaust gas turbine (22) to the outlet passage side of a compressor (21) is absent, the engine brake can be effectively applied to the diesel engine during running at high speeds and running on adownward slope.