To reduce the harmful matter emission amount of an internal combustion engine and increase the output of the internal combustion engine by controlling at least one adjusting means which can be driven depending on the detected oxygen content to change the concentration of sucked air and oxygen condensing air.

An intake path 20 of an internal combustion engine 10 is provided with an air purifier 22 on its inlet side to inhibit dust from reaching the internal combustion engine 10, and a throttle valve 24 controlled by a control device 40 is arranged in the middle part of the intake path 20. As a device for condensing the oxygen content in intake air of the internal combustion engine 1, a condensing path 30 connected to the intake path 20 is provided, and similarly provided with a throttle valve 34 controlled by the control device 40. In this condensing path 30, a film 32 having permeability especially to oxygen molecules is disposed, thereby condensing oxygen when air is sucked through the film under the action of intake negative pressure, and the condensed oxygen is supplied together with sucked air to the internal combustion engine 10.