An ink jet printer head and a piezoelectric actuator for the head, in which energy conversion efficiency from an electric energy to a mechanical vibration energy is high, and its mechanical displacement of a vibration transferring plate is large, are provided. An ink jet printer head having a piezoelectric actuator provides an upper electrode, a piezoelectric plate, a lower electrode, a vibration transferring plate, and a pressure chamber, whose shape is a rectangle and its length ratio of the short side to the long side is 0.8 or more and 1.0 or less. And the piezoelectric plate having the lower electrode is bonded to the vibration transferring plate at a position within the inside wall of the pressure chamber and the center positions of the piezoelectric plate and the pressure chamber are set to be almost equal. Therefore, the energy conversion efficiency of the piezoelectric plate can be utilized in its maximum, and the piezoelectric actuator whose mechanical displacement is large can be realized. Consequently, the industrial value of the ink jet printer head becomes large by applying the piezoelectric actuator to an ink jet printer.