A temperature measuring apparatus measures the temperature of a medium according to the propagation time of ultrasonic waves propagated for a predetermined distance through the medium. The apparatus has a transmitter 11, 12 for transmitting ultrasonic waves having a fixed frequency at predetermined timing and a receiver 13, 14, 15 for receiving the ultrasonic waves and providing a received signal. A delay time detector 24 detects, in response to the received signal, a delay time between the transmission and reception of the ultrasonic waves and a wave number calculator 21 calculates an integral wave number according to the delay time and a period of the ultrasonic waves. A phase difference detector 25 detects a phase difference between the transmitted and received ultrasonic waves modulating the received signal with a frequency that is sufficiently higher than the frequency of the ultrasonic waves. An operation unit 21 calculates the propagation time of the ultrasonic waves according to the wave number and phase difference, and according to the propagation time, the temperature of the medium. This apparatus has a wide range of measurable temperatures, and is capable of precisely measuring the temperature of a wide area of a medium.