There is provided a flat type piezoelectric ultrasonic motor for generating a rotation force to a rotor by ultrasonic vibrations. In the piezoelectric ultrasonic motor, a vibrator is formed of elastic material at a given size, a plurality of piezoelectric plates are attached on the vibrator, a rotor is drivingly connected to the vibrator thereby to be rotated, and a power source unit applies driving voltages to the piezoelectric plates. The piezoelectric plates are arranged on quartered surface areas of the vibrator in such a way that longitudinal axes of neighboring piezoelectric plates form a given angle therebetween and opposing piezoelectric plates have opposite polarizations, and the power source unit applies alternating driving voltages to the piezoelectric plates thereby to make the rotor rotated, thereby making it possible to simplify the piezoelectric plates' shape, minimize its size and enhance its driving efficiency.