PURPOSE: To measure the flow rate accurately regardless of the magnitude of the flow rate, by arranging a plurality of flowmeters having different measuring ranges in series in the flow of liquid, and bypassing only the required flowmeter in response to the flow rate.

CONSTITUTION: For example, when the flow rate is 80m3/min or less, a flow rate regulating valve 5 is closed, the entire liquid flows in the first flowmeter 1, and the flow rate is measured. Meanwhile, when the flow rate exceeds 80m3/ min, the valve 5 is opened, and the fluid which is joined with the fluid through a bypass pipe 4 is measured by the second flowmeter 3. By the arrangement of the flowmeters in response to the flow rate, the flow rate up to 5,000m3/min is accurately measured in the entire range.