PURPOSE: To vary the capacity of a measurement container.

CONSTITUTION: A fluid to be measured is filled into a same measurement container regardless of the level of the shape of a horizontal section, the differential pressure between the inside of the measurement container and the fluid to be measured and unloaded is detected, and then the flow rate of the fluid is measured in superposition based on the amount of change. The measurement container consists of a basic container 11 and one or more connection containers 12 which are laid out inside the basic container 11 and can communicate with the basic container 11 through the inside. Then, by selectively connecting the connection containers 12 to the basic container 11, a different horizontal section is created and a measurement range is variable. Also, one or more elimination rod which can be loaded or unloaded are provided in the measurement container or a plurality of unit measurement containers with the same or different diameter may be provided.