An accumulator back pressure control device for an automatic transmission, the accumulator back pressure is regulated when a shift valve is switched. For example, when shifting up from the first to the second speed, when the second brake starts to engage by switching of the shift valve, rotation speed of the input side varies by the change of the output torque of a trans­mission gear mechanism. By detecting the rotation change by a sensor, the pressure control valve (linear solenoid) is control­led by signals from a control unit so that the control pressure from the pressure control valve is controlled at required moment. By this, for example, a control valve, by the change of hydraulic pressure at the control pressure port, regulates the pressure of oil source (line pressure port) and applies such regulated pressure to the back pressure chamber of the B2 accumulator to change the back pressure of the accumulator. When shifting from the second to the third speed, the back pressure of the C2 accumulator is controlled, and when shifting from the third to the fourth speed, the back pressure of the B0 accumulator is controlled. When down shifting, the operations are performed in same manner as the above.