PURPOSE: To measure the flow rate of exhaust in a flow-back pipe by providing O2 sensors in the exhaust pipe and an exhaust flow-back pipe of the internal combustion engine respectively and by operating the difference between detections by both sensors.

CONSTITUTION: The intake and exhaust pipes 2 and 3 of the main body 1 of the engine are communicated with each other by the flow-back pipe 5 having an exhaust flow-back controlling valve 6 and a part of exhaust gas is made to flow back to an intake system. To a control device 7 various signals 13 for rotational number, negative pressure, etc. are inputted, whereby the opening degree of the controlling valve 6 and a fuel injection valve 11 is determined. Moreover, the exhaust pipe 3 and the flow-back pipe 5 are provided with the O2 sensors 8 and 12 by one respectively, and the signals therefrom are operated by the control device 7, whereby the flow rate of the gas flowing through the flow-back pipe 5 is measured. Furthermore, the flow-back pipe 5 is provided with a temperature sensor 9, whereby the compensation of the flow rate of the exhaust gas flowing through the flow-back pipe 5 is conducted according to the temperature.