The feasibility of ultrasonic in-situ measurement of friction surface temperature has been examined. The ultrasonic thermometry that is a method providing nondestructive temperature measurements by ultrasound is applied to temperature measurements of a friction surface at which temperature rise occurs due to friction, and an attempt is made to demonstrate in-situ monitoring of transient variations in the friction surface temperature and temperature distribution beneath the surface. Those temperatures are quantitatively determined by a combined method consisting of ultrasonic pulse-echo measurements and a finite difference calculation for estimating one-dimensional temperature distributions along the direction of ultrasound propagation. To demonstrate the practical feasibility of the method, the ultrasonic pulse-echo measurements at 2 MHz are performed for an acrylic resin plate of 10 mm thickness whose single side is being heated by friction with a felted fabric plate. The temperature profile near friction surface and its transient variation are measured during the friction heating under different applied loads to the friction surface. It has been observed that the temperature at friction surface increases quickly and significantly just after the friction started, and the maximum temperature rise at the friction surface increases markedly with the applied load. Thus, in-situ measurement of friction surface temperature by the ultrasonic thermometry has successfully been demonstrated.