PURPOSE: To perform a precise measurement of an amount of air bubbles in a liquid metal such an Na for use with a cooling for a fast breeder, by a method wherein an ultrasonic generating means, and a means which always compares an ultrasonic receiving means and receiving sound pressure with a specified sound value, are mounted in a liquid metal.

CONSTITUTION: To measure the presence of air bubbles in a liquid metal Na for use with, for example, a fast breeder, an ultrasonic generating element 15 and an ultrasonic receiving element 16 are mounted to a lower part of an overflow column 14 for liquid Na. The element 15 is oscillated by an oscillator 17, and an ultrasonic wave with a specified sound pressure is normally emitted to the element 16. A receiver 18 converts a sound pressure, received by the element 16, into an electric signal. In case air bubbles exists in the liquid Na, a part of transmitting sound waves are reflected by air bubbles, and a receiving sound pressure reduces. Thus, through comparision of the receiving sound pressure with said specified sound pressure, a comparator 19 is caused to check for the pres-ence of air bubble, and in case the sound pressure is low, an alarm signal 20 is generated. This prevents the occurrence of abnormal vibration of a liquid Na circulating pump due to the presence of the air bubbles and the occurrence of troubles such as the formation of a part, where there is not cooling material existing, resulting from the rapid expansion of the air bubbles in a reactor core.