A method and device for automatically controlling hydraulic pressure in a hydraulic cylinder for hot runners of pressure regulating injection molding machines is disclosed. This device is provided with a pressure regulating unit, having both a pressure regulating spring and a pressure regulating head, at each of two hydraulic ports formed at the upper and lower portions of the cylinder's sidewall, and makes one of the two units open an associated hydraulic port when a pressure, exceeding a reference level predetermined by both the spring force of the pressure regulating spring and the cross-sectional area of the pressure regulating head, acts on the piston of the cylinder. A predetermined quantity of pressurized oil is discharged from the cylinder through the open port and thereby maintains desired constant pressure acting on the piston in the cylinder. This device has a simple construction, and reduces the number of required solenoid valves of the cylinder from four to two, thus being easily produced at low production cost. The device also automatically regulates the hydraulic pressure inside the cylinder, and forces the actuation oil to continuously flow from the cylinder, thus preventing carbonization of the oil in the cylinder.