There is a case where a self-excited pressure variation is generated in low-pressure pipeline systems using an automatic pressure-reducing valve. For example, the pressure acting on the tube and valve is higher than the expected pressure and results in damage to the equipment. This study aims to explain the upstream and downstream pressure variations of the valve by carrying out pressure measurements in an actual pipe, in order to identify the mechanism of the self-excited variations and take effective countermeasures. For a reducing valve set up in the middle of the pipeline, the results indicated that the pressure variations that occur soon after opening the valve propagate upstream and downstream without attenuation, maintaining a constant amplitude, and generate continuing pressure variations that resemble self-excited oscillations. In addition, numerical simulations based on the wave model are conducted to evaluate the relationship between the response of an automatic pressure-reducing valve and the hydraulic transient phenomena in a pipeline system. If the pressure-reducing valve is adjusted for high-sensitivity, pressure variation due to external disturbances in the terminal region (downstream of the valve) may increase because of the period of pressure oscillation in the conduit of the upstream and downstream parts of a control valve.