PURPOSE: To properly coordinate air fuel ratio feed-back control with evaporated fuel introduction by increasing a purge amount of the evaporated fuel when a signal of an O2 sensor is reversed from a rich condition to lean one.

CONSTITUTION: An O2 sensor 20 for generating the output based on the oxygen concentration in exhaust gas is provided in an exhaust path 19. Also, evaporated fuel trapped in a canister 23 is introduced through a supply path 24 into a surge tank 17 of an intake path 9 to be controlled by a duty solenoid valve 25. A control unit 25 outputs an opening signal to the duty solenoid valve 25 for the adjustment of the evaporated fuel to reduce a purge amount when the output of the O2 sensor 20 is reversed from lean condition to rich one and increase the purge amount when the output is changed from rich condition to lean one. Thus, the control of purge introduction matching with the feed-back control of air fuel ratio can be performed so the stable air fuel ratio feed-back can be controlled while the purge of the evaporated fuel is carried out successfully.