A single-shot injection system for Fuel Injection was developed to investigate basic aspects of the spray combustion phenomenon. In this system, a new solenoid three-way valve is used to achieve a constant rail pressure state during the injection period and the same characteristics as that of an actual injection system can be realized. From a series of injection pressure measurements, the effectiveness of the rail pressure control on the single-shot injection characteristics are shown. This system offers a basis for the systematic study of a fuel injection system.