PURPOSE: To reduce occurrence of HC, white smoke and black smoke in the apparatus in the caption by providing a sub-conduit intake air throttle valve on the sub-intake air conduit of a diesel engine and maintaining the air fuel ratio in the most suitable condition by controlling the open degree by the aid of an air fuel ratio sensor provided on an exhaust pipe.

CONSTITUTION: The intake air conduit 2 of a diesel engine 1 branches in a main intake air conduit 2a provided with a main conduit intake air throttle valve 3a interlocking with an accel pedal 6 and a sub-intake air conduit 2b provided with a sub-conduit intake air throttle valve 3b interlocking with a vacuum actuators 11. And, each detected valve of an air fuel ratio sensor 5 provided on an exhaust pipe 4, an accel open degree sensor 6a, a rotational speed sensor 7 and a cooling water temperature sensor 8 or the like is inputted into the control unit 20 which controls a vacuum actuator 11 via a pressure control valve 10, and the control of the actuator 11 is performed so as to get target air fuel ratio in accordance with the accel open degree, rotational speed and cooling water temperature.