To accurately detect the liquid level and liquid material information on a liquid tank by detecting the liquid level, etc., based on the pressure data which are obtained from a pair of upper and lower pressure sensors set in the liquid tank.

At least a pair of 1st and 2nd pressure sensors 3 and 4 are attached to the side part 1a of a liquid tank 1 via the flanges 3a and 3b. When the level H of the liquid 2 is calculated, the 1st pressure data P1 of the sensor 3 is shown as P1=ρg(H-H1) with the 2nd pressure data of the sensor 4 shown as P2=ρg(H-H2) respectively. The level H can be detected as H=(P2H1-P1 H2)/(P2-P1). In these expressions, ρ shows the density of the liquid 2 with (g) showing the gravitational acceleration, H showing the liquid level height and H1 and H2 showing the heights measured between the bottom of the tank 1 and both sensors 3 and 4 respectively. As a result, an accurate liquid level is detected despite the vibration of the liquid level.