A piezoelectric actuator unit which undergoes less change in the amount of displacement and shows high durability in continuous operation under a high voltage and a high pressure over a long period of time is provided.The piezoelectric actuator unit including a multi-layer piezoelectric element having piezoelectric layers and metal layers with the piezoelectric layers and the metal layers being stacked one on another, and a case which houses the multi-layer piezoelectric element, wherein at least one of the plurality of metal layers is stress relieving layer which consists of a plurality of partial metal layers that are scattered throughout the entire region between two piezoelectric layers which adjoin the partial metal layers in the direction of stacking, and voids, and a peel-off section is formed at least in a part of the interface between the stress relieving layer and the piezoelectric layer that adjoins therewith.