To cope with an abrupt increase in a gas flow at the onset of measurement without damage.

A float 12 is inserted into a tapered tube 11. A stopper 16 is placed at the upper part of the tapered tube 11. A space 17 is provided above the stopper 16. A throttle hole 19 is formed in a partition 18 at the top of the space 17. A throttle valve 20 which can float vertically and rises to close the throttle hole 19 when there is an abrupt increase in the gas flow of fluid under measurement is placed within the space 17. When a valve in a pipeline is open at the onset of measurement, and the gas abruptly flows into the tapered tube 11, the float 12 also rises, and the throttle valve 20 also rises abruptly and the throttle valve 20 closes the throttle hole 19. Thereby an inflow of the gas is prevented to reduce an impact value of the float 12 on the stopper 16 and the tapered tube 11 is prevented from being damaged.