PURPOSE: To measure an extensively variable water level at high accuracy, by holding an ultrasonic-wave transmitting and receiving apparatus available for vertical displacements at the specified distance from a water level.

CONSTITUTION: At a jointing bracket 9 of a supporting strut 6 where ultrasonic- wave transmitting and receiving apparatus 2 and theremometer 10 are connected with a supporting body 4 through a connecting rod 5, the title apparatus 2 and the thermometer 10 are free to move up and down. The distance l between the apparatus 2 and a water level is held within a range; 2m<l<(2+L)m, where L represents natural numbers 1W10. An ultrasonic-wave pulse is applied to the water level 1 and the water level is obtained from transmission time of pulse returning from the water bottom 7 after reflection and corrected by the atmospheric temperature and sound velocity. As the distance between the apparatus 2 and water level 1 is kept within the constant range, measurement of extensively variable water level can be available with high accuracy.