Blood pressure measuring technique using a pneumatic cuff is most important noninvasive method to determine the blood pressure. In this method cuff-pneumatic pressure is at first set higher than systolic blood pressure and then decreased gradually. In this process, blood pressure is determined by start and end of Korotokoff sound or characteristic pattern of cuff-pressure amplitude of oscillation. As the signals of sound or pressure oscillation are obtained intermittently, the cuff-pressure decreasing velocity is needed to be controlled at about 3 mmHg/s to assume the accuracy of measurement. But the velocity is affected by a size of an arm or its stiffness and, hence, it does not become constant when a simple restriction is used to release air of the cuff. In this research a control technique to decrease the cuff-pressure at constant velocity using a simple pneumatic control valve is proposed. The analytical results obtained in this research agreed well with experimental results, and usefulness of this method is known.