PURPOSE: To improve the precision of liquid level measurement by calculating the height to the liquid surface of a solution from the proportional relation among the total pressure of the bottom surface of a tank which is measured by a static pressure sensor, the pressure difference between the upper and lower parts of the tank which is measured by a differential pressure sensor, and a predetermined distance.

CONSTITUTION: This measuring instrument is equipped with an arithmetic processing part 5 which calculates the height up to the liquid surface of the solution from the proportional relation among the total pressure obtained by the static pressure sensor 2, the pressure difference measured by the pressure difference sensor 1, and the predetermined distance, the differential pressure sensor 1, and the static pressure sensor 2. Then the pressure of two opening parts 7 and 8 provided at specific positions of different level in a container such as the tank 6 is guided to the differential pressure sensor 1, which measures their pressure difference to obtain a signal indicating the density of the liquid in the container. Further, the pressure of the opening part 7 positioned at a lower position between the opening parts 7 and 8 is guided to the static pressure sensor 2, which obtains a signal corresponding to the liquid level of the container 6; and the liquid level signal is corrected with the density signal of the liquid based upon the differential pressure sensor signal to obtain a liquid level signal from which the influence of the density of the liquid is excluded.