A calibration method for a liquid flowmeter comprising the steps of: providing a first tank for receiving said liquid to be measured and a liquid storage tank connected to said first tank; providing a liquid delivering line for conveying the liquid from said liquid storage tank to an external device; providing a weighing sensor for weighing the liquid contained in either the first tank or the liquid storage tank, during performing the calibration for the flowmeter: disconnecting the liquid delivering line in a conveyance direction downstream of the liquid flowmeter from the external device; connecting said liquid delivering line in the conveyance direction downstream of said liquid flowmeter to a calibrating line connected to said first tank so that the liquid from said liquid storage tank is delivered through the liquid delivering line so as to pass through the flowmeter back to said first tank; and determining an estimated flow rate of the conveyed liquid by calculating the change in weight of the conveyed liquid measured by said weighing sensor during a predetermined time interval, the method being characterized in that, during performing the calibration, the flow rate of the liquid from said liquid storage tank to said first tank passing through the flowmeter is controlled by a controller so as to be substantially constant and substantially equal to the flow rate during the actual operation, throughout the calibration, using a feedback signal from the flowmeter and in that the reading from the flowmeter is adjusted so that it is in conformity with said estimated flow rate. providing a first tank for receiving said liquid to be measured and a liquid storage tank connected to said first tank; providing a liquid delivering line for conveying the liquid from said liquid storage tank to an external device; providing a weighing sensor for weighing the liquid contained in either the first tank or the liquid storage tank, during performing the calibration for the flowmeter: disconnecting the liquid delivering line in a conveyance direction downstream of the liquid flowmeter from the external device; connecting said liquid delivering line in the conveyance direction downstream of said liquid flowmeter to a calibrating line connected to said first tank so that the liquid from said liquid storage tank is delivered through the liquid delivering line so as to pass through the flowmeter back to said first tank; and determining an estimated flow rate of the conveyed liquid by calculating the change in weight of the conveyed liquid measured by said weighing sensor during a predetermined time interval, the method being characterized in that, during performing the calibration, the flow rate of the liquid from said liquid storage tank to said first tank passing through the flowmeter is controlled by a controller so as to be substantially constant and substantially equal to the flow rate during the actual operation, throughout the calibration, using a feedback signal from the flowmeter and in that the reading from the flowmeter is adjusted so that it is in conformity with said estimated flow rate. disconnecting the liquid delivering line in a conveyance direction downstream of the liquid flowmeter from the external device; connecting said liquid delivering line in the conveyance direction downstream of said liquid flowmeter to a calibrating line connected to said first tank so that the liquid from said liquid storage tank is delivered through the liquid delivering line so as to pass through the flowmeter back to said first tank; and determining an estimated flow rate of the conveyed liquid by calculating the change in weight of the conveyed liquid measured by said weighing sensor during a predetermined time interval, the method being characterized in that, during performing the calibration, the flow rate of the liquid from said liquid storage tank to said first tank passing through the flowmeter is controlled by a controller so as to be substantially constant and substantially equal to the flow rate during the actual operation, throughout the calibration, using a feedback signal from the flowmeter and in that the reading from the flowmeter is adjusted so that it is in conformity with said estimated flow rate.