To monitor mass and density of a pressurized fluid by providing density such as a density sensor moves to a first position in a container when fluid density is prescribed density and moves to a second position in the container when the fluid density is lower than the prescribed density.

An indicator 34 displays whether fluid density in a container 10 is lower or higher than density of a density sensor 24 on the basis of a frequency of a transmitter 30 decided by a frequency measuring device 32. When the fluid desnity is smaller than ?density of the density sensor 24, the density sensor 24 is positioned adjacently to the bottom part of a sensing chamber 18. When the fluid density is larger than density of the density sensor 24, the density sensor 24 floats up to a position adjacent to the top part of the sensing chamber 18. The indicator 34 has output for showing whether the fluid density is lower or higher than density of the density sensor 24 to thereby monitor density and mass of fluid pressurized in the container 10.