The piezoelectric actuator includes a piezoelectric film between two electrode layers and a diaphragm. Assuming that: each elastic coefficient of all materials is isotropic and a distortion amount of the piezoelectric film by an electric field is isotropic in all in-plane directions; a point located on a diaphragm surface and having a maximum displacement when a predetermined electric field is applied to distort the piezoelectric film, is expressed by PδMAX; and a point located on a circumference of a reference-circle having PδMAX as a center and having a minimum difference in displacement from PδMAX is expressed by PδA, the diaphragm has a shape capable of determining an axis A1 set in a straight-line joining PδMAX and PδA, the diaphragm comprises a single-crystalline-material in which a plane orthogonal to A1 and perpendicular to an axis A2 on the diaphragm surface, is a {110}-plane, and the piezoelectric film is a {100}-single-orientation film.