To measure a temperature of a fluid by simple constitution.

This temperature measuring instrument for measuring the temperature of the fluid passing through a conduit 9 is provided with a pair of ultrasonic transceiver 10a, 10b arranged in an outer circumferential part of the conduit 9, and a computing part 5 for computing a propagation time and a sonic velocity of an ultrasonic wave propagated inside the conduit 9, and a propagation time and a sonic velocity of the ultrasonic wave propagated in the fluid. The ultrasonic wave is propagated bidirectionally between the pair of ultrasonic transceiver 10a, 10b, and is received therebetween each other, and the computing part 5 specifies the temperature of the fluid, based on a computed sonic velocity value of the ultrasonic wave.