We propose a phase-controlled traveling-wave-type ultrasonic motor (USM) driven in the constant temperature mode. By setting the phase difference between two sinusoidal driving signals fed into USM zero, a thermally-equilibrium condition of USM is attained without rotation. We call such an operation mode idling. Because the resonance frequency of the USM depends on the temperature, the rotational speed is decreased with increasing the temperature, which has been made the frequency control of USM somewhat difficult. Unlike the conventional frequency-controlled USM, a phase-controlled one with idling is free from the temperature change and is controllable with ease. We have constructed a feedback loop system for the phase-controlled USM and evaluated its performance in comparison with the conventional frequency-controlled one.