mechanical structure is concerned and is provided with a turbocharger 1 equipped with a variable boost pressure mechanism an intake passage 2 is connected to a compressor end of the turbocharger 1 and an exhaust passage 3 is connected to a turbine end of the turbocharger 1 an air cleaner 4 is connected to the upstream end of the intake passage 2 and an intake control valve 5 for controlling the flow rate of the fresh air that flows into a combustion chamber and a swirl control valve 6 for increasing the flow speed of the intake flow by restricting the cross sectional area of the passage under a low speed partial load condition are provided in appropriate parts of the intake passage 2 to the downstream end of the exhaust passage 3 is connected an exhaust cleaning system 10 comprising a diesel oxidation catalytic converter doc 7 a diesel particulate filter dpf 8 for removing particulate substances such as soot and a lean nox catalytic converter lnc 9 discussed above that are connected in that order the swirl control valve 6 and a part of the exhaust passage 3 immediately downstream of the combustion chamber are connected to each other via an exhaust gas recirculating egr passage 11 the egr passage 11 comprises a cooler passage 11 a and a bypass passage 11 b that bifurcate at a switching valve 12 provided at an.