PURPOSE: To effect high accuracy measurement of extensively varying water level, by holding an ultrasonic-wave transmitting and receiving apparatus available for vertical displacements in the specified range of distances from the water and computation processing the transmission time of reflected pulses from the water bottom.

CONSTITUTION: By a supporting member 8 an ultrasonic-wave transmitting and receiving apparatus 2 mounted free to move up and down on a strut 6 through a connecting rod 5, a distance l0 between water level and apparatus 2 is held to represent a relation; 2m<l0<(2+L)m; wherein L=Natural number 1W10. Ultrasonic-wave is transmitted from the wave transmitting and receiving apparatus 2 on the water surface 1, and from the transmission time of reflected pulses from the water bottom 7, the water level H is calculated from the following equation; H=D-CTt/2=D-l0t/t0, CT=l0/t0, however, D stands for a distance (m) between zero water level and wave transmitting and receiving apparatus, CT sound velocity at atmospheric temperature T, t0 transmission time of pulse between water level and wave transmitting/receiving apparatus, (t) reciprocating transmission time. As l0 is held within a constant ronge, high accuracy measurement of extensively varying water level becomes available.