To prevent deforming and damaging of a thermal expansion absorbing member due to preload, by preventing acting of the preload, which is applied to a piezoelectric element actuating an open/close valve via a connection member as the thermal expansion absorbing member, on the connection member.

A piezoelectric element 80 constituting an actuator 51 of this injector is formed of a ceramic material stored in a case 81. The piezoelectric element 80 is used in a state in which preload F is previsouly applied by the case 81. Since the preload F is internal force in the actuator 51, the preload F is prevented from acting on a connection member arranged at an output side of the piezoelectric element 80 and serving as a thermal expansion absorbing member having high thermal expansion and low hardness such as aluminum compared with an injector body, and an operation of the piezoelectric element 80 is accurately transmitted to the output side without deformaing and damaging the connection member.