To stably reduce the common rail pressure using an injector operated by a piezo-actuator in a common rail type fuel injector.

A valve element 123 for relieving back pressure of a needle 121 of the injector 1 to a low pressure source is pressed to be driven by a piezo-stack 127 to achieve half-lift, whereby with the needle 121 still in the seated state, fuel is relieved from a back pressure chamber 106 to reduce the common rail pressure. In pressure reduction, piezo-stack voltage is raised to a half-lift enabling value in a designated control cycle, and the higher the common rail pressure is, and the higher the lowering speed of injection start voltage is, the percentage of the charging state period in the control cycle can be decreased, whereby even if the lowering speed of the injection start voltage is high, the injection start voltage is not below the actual piezo-stack voltage.