In a gear shift control unit for an automatic transmission, when an up-shift switch or a down-shift switch of a paddle shift switch is turned ON, a gear shift mode is set to a temporary manual shift mode, and a target transmission gear stage is shifted up or shifted down. Subsequently, a manual assist control is executed. In the manual assist control, an acceleration is calculated from a temporal differentiation of a vehicle speed. After it is detected that the acceleration is larger than or equal to an acceleration determination threshold, an accelerator pedal return acceleration is calculated from a double temporal differentiation of an accelerator opening degree. When the accelerator pedal return acceleration is smaller than or equal to another acceleration determination threshold, the current target transmission gear stage is automatically shifted down.