For an improvement of fuel consumption and emission purification, engine operating parameters, such as a valve operation, become variable. Therefore, an engine operation at a transient state was increased and an accuracy was increasingly required to control an air fuel ratio at transient state. This paper investigated an adaptive gain control system which was respectively equipped to the feedforward and the feedback controller. Then, a robustness against operating condition and system parameter were also investigated by comparing with some single side adaptive controller. A high accuracy control result and robustness of the controller were shown by the experimental results.