An automatic transmission has an electronic control unit and a hydraulic control unit. The electronic control unit outputs an engagement command pressure to a solenoid disposed in a hydraulic control unit to generate a shelf pressure to upshift the automatic transmission. The engagement command pressure is corrected downwardly when the maximum rate of the gear ratio change is larger than the rate of change of a thrust-up determination gear ratio, and upwardly when the maximum rate of the gear ratio change is smaller than the rate change of a prolongation determination gear ratio. By correcting the engagement command pressure using the maximum rate of the gear ratio change, a proper output shaft torque wave form can be obtained so that a fluctuation of an output shaft torque during a torque phase while upshifting has no strong influence on the acceleration of a vehicle to achieve a comfortable shift feeling.