This paper proposes gain scheduled H_∞ flight control design method, which can satisfy specified stability margin. The parameter changes due to flight conditions are described by LPV (Linear Parameter Varying) model, and gain scheduled controller is designed by LMI (Linear Matrix Inequality) theory. Stability margin requirements can be described as a kind of mixed sensitivity H_∞ problem with constant weighting.Therefore, stability margin requirements and another design requirements can be simultaneously satisfied by solving LMI. The effectiveness of this design method is shown by designing longitudinal flight controller for ALFLEX (Automatic Landing FLight EXperiment).