To acquire a flowmeter and a flow measuring method having high accuracy by using an inexpensive resistance tube having dispersion in a shape or a size.

This flowmeter is constituted of the resistance tube 11 for sending measuring gas, a pressure sensor 12 for detecting the upstream side pressure of the resistance tube 11, a CPU 13 and a storage device 14 for determining a parameter for a differential pressure/flow rate conversion formula by calibration, calculating the flow rate based on the differential pressure/flow rate conversion formula, and outputting the data, an output device 15 for displaying the flow rate value, and an input device 16 for inputting a command at the calibration time. In the calibration, calibration flow rates of three or more values are allowed to flow, and the differential pressure/flow rate conversion formula is determined by a least square approximation method wherein the sum of squares of each difference between each flow rate and each flow rate value determined by substituting each differential pressure measured at that time for the differential pressure/flow rate conversion formula expressed by a second or higher degree expression is minimized.