Abstract: A fuel injector wherein a piston driven by the extension and contraction of a piezoelectric actuator changes the pressure of a working fluid in a variable pressure chamber and back pressure chamber so as to drive a needle valve to open and close a fuel injection hole, wherein undesirable secondary injection after a main injection in prevented by providing a throttle portion for narrowing the communicating passage between the variable pressure chamber and back pressure chamber when the piston moves forward and the needle valve closes the fuel injection hole.