A bypass conduit (23) connects an outlet passage of a compressor (21) of an exhaust gas turbocharger (20) and an inlet passage of an exhaust turbine (22), and a bypass valve (24) is provided therein. When the operating condition of a diesel engine (1) is found to be in a low-speed and high-load region, the bypass valve (24) is controlled by means of a valve controller to adjust the bypass conduit (23) so as to open it. Then, the charge air partly flows into an exhaust conduit (4) to increase the rotational speed of the exhaust turbine (22) and hence the charge air flow rate raises the output of the diesel engine (1). Additionally, as the charge air flow rate is increased, the operating condition of a compressor (21) is prevented from approaching the surging range.