In order to increase the lifetime for a piezoactuator of a fuel injector and to make possible a reliable seal of the piezoactor especially for comparatively thin head plates as well a gas exchange passage running through an actuator housing (52, 54) is provided, where a sealing unit (2) placed on the head arrangement (16, 18) is provided for sealing the piezoactor (12) which features a carrier (28) on which a sealing material (30) is arranged which forms a seal around the piezo contact pins (14) and the head arrangement (16, 18) and on which a microporous gas exchanger (26) to create gas permeability of the sealing unit (24) is integrated such that any fluid which has penetrated through to the gas exchanger (26) can flow out of the gas exchanger (26) under the force of gravity.