To provide a flowmeter capable of performing conversion including all error influences such as an instrumental error, a mounting influence or a using condition independently of flow velocity distribution in a measuring tube, when finding a cross-sectional average flow velocity or the flow rate from a partial flow velocity by conversion.

When showing an insertion type thermal flowmeter 1 as an example, actual flow calibration is performed with a standard flowmeter 2 as preparation for actual flow measurement in the thermal flowmeter 1. The actual flow calibration is performed in an actual using condition of the thermal flowmeter 1 or in a condition similar thereto as follows: the standard flowmeter 2 is disposed temporarily in a flow tube 4 where the thermal flowmeter 1 is disposed; the average flow velocity or the flow rate of the measuring fluid flowing in the flow tube 4 is measured by the standard flowmeter 2; the partial flow velocity is measured simultaneously by the thermal flowmeter 1; both data are correlated with each other; and the result is stored in a storage means provided inside the thermal flowmeter 1 as data for conversion and the calibration is finished. Thereafter, when the standard flowmeter 2 is dismounted and actual measurement is performed by the thermal flowmeter 1, the data stored in the storage means are used for the conversion from the partial flow velocity into the flow rate or the cross-sectional average flow velocity.