To provide a liquid volume monitoring device and a monitoring method therefor to detect an amount of fuel storage in a vehicular fuel tank.

A second and third pressure sensors (PT2, PT3) are placed close to the bottom of the tank 10, one sensor being placed apart from the other by a predetermined distance delta h in the vertical direction. Further, a first pressure sensor PT1 is placed close to the top of the tank to detect vapor pressure above the liquid volume level in the tank. Apparent liquid pressure is calculated from the pressure difference between uppermost and lowermost sensors (PT1, PT2) in the tank, and density is calculated from the pressure difference between a pair of sensors (PT2, PT3) mounted at the bottom. The calculated apparent liquid pressure and density are used for the calculation of the liquid level height in the tank, and the fuel storage volume is determined by a look-up table for the liquid level height and a corresponding liquid volume.