A Steam Injector (SI) is a simple, compact and passive pump. We are developing this innovative concept by applying the SI system for core injection system in emergency core cooling systems (ECCS) to further improve the safety of nuclear power plants. Passive ECCS in nuclear power plants would be inherently very safe and would prevent severe accidents by keeping the core covered with water (Severe accident-free concept). The passive core injection system (PCIS) driven by high-efficiency SI is a system that, in an accident such as a LOCA, attains a higher discharge pressure than the supply steam pressure used to inject water into the reactor by operating the SI. This report describes the experiments and the analytical simulation on a SI-driven PCIS for innovative-simplified nuclear power plant. In addition, we conducted the analytical simulations of SI, which grew in size for the actual nuclear power plant.