Pressure regulating system for use in an automatic transmission including a fluid coupling, a belt drive continuously-variable speed transmission mechanism having an input pulley and an output pulley which are rotatably interlocked with a V-belt and a reduction ratio control system for controlling the reduction ratio of the belt drive continuously-variable speed transmission mechanism. The pressure regulating system includes a regulator valve capable of regulating the discharge pressure of an oil pump to provide a line pressure, a reduction ratio detecting valve and a throttle pressure valve. The reduction ratio detecting valve receives the line pressure and provides a pressure which is proportional to the displacement of the movable flange of the output pulley with respect to the corresponding fixed flange. The throttle pressure valve receives the line pressure and provides a first throttle pressure which is proportional to the degree of throttle opening of an internal-combustion engine and receives the reduction ratio pressure and provides the same reduction ratio pressure as a second throttle pressure only when the degree of throttle opening exceeds a predetermined value. The regulator valve receives the reduction ratio pressure, the first throttle pressure and the second throttle pressure to regulate the line pressure.