Fuel injection system

A fuel injection system provided with a three-way valve able to selectively connect a back pressure control chamber formed on an inside end face of a needle valve and an intermediate chamber of a booster piston for increasing an injection pressure to a high pressure fuel feed passage or low pressure fuel return passage and, control for opening and closing a needle valve and control for increasing the injection pressure by the booster piston are performed by using the fuel passage switching action by the three-way valve, wherein a pressure switching chamber constantly connected to either the back pressure control chamber or intermediate chamber is formed in the three-way valve, the high pressure fuel feed passage is open to one side of the pressure switching chamber, a first valve element for controlling the opening and closing of the opening of the high pressure fuel feed passage is provided, the low pressure fuel return passage is open to the other side of the pressure switching chamber, a second valve element for controlling the opening and closing of the opening of this low pressure fuel return passage is provided, the three-way valve is provided with a pressure control chamber, fuel pressure in the pressure control chamber is controlled so as to control a pressure difference of fuel pressures acting at the two ends of the first valve element in an axial direction of the first valve element and a pressure difference of fuel pressures acting at the two ends of the second valve element in an axial direction of the second valve element so that when switching the destination of either the back pressure control chamber or intermediate chamber from the high pressure fuel feed passage to the low pressure fuel return passage, the state where the first valve element is open and the second valve element is closed is changed through a state where the first valve element and second valve element are both closed to a state where the first valve element is closed and the second valve element is open and so that when switching the destination of either the back pressure control chamber or intermediate chamber from the low pressure fuel return passage to the high pressure fuel feed passage, the state where the first valve element is closed and the second valve element is open is changed through a state where the first valve element and second valve element are both closed to a state where the first valve element is open and the second valve element is closed, and the other of the back pressure control chamber or intermediate chamber is connected with the pressure switching chamber when second valve element is open or is constantly connected with the pressure control chamber.