PURPOSE: To make it possible to measure water temperature continuously and to perform the measurement of the water temperature in a broad range in a short time, by providing ultrasonic-wave transmitting and receiving devices at specified positions on the bottom of a ship, and measuring the states of the transmitted and received waves.

CONSTITUTION: Ultrasonic waves f0 are continuously transmitted from the bottom of a navigating ship at a depression angle β. The ultrasonic waves f0 are reflected by the sea bottom M and received by receiving devices 3A, 3B and 3C, respectively. However, the distribution of the sound speeds is not uniform, and the speed increases in proportion to the depth of water. Therefore, the sound speeds do not become constant. The propagating path of the sound wave is continuously refracted. In general, in a dopple sonar, the depression angle of the transmitted ultrasonic wave is sufficiently large so that total reflection is not yielded at an interface. Therefore, the sound wave is refracted downward primarily owing to the temperature gradient, reflected by the sea bottom, and returned. Thus, the continuous measurement of the water temperature can be performed. Since this operation is continuously performed, the measurement of the water temperature in a broad range can be performed in a short time.