This paper presents a new real time measuring method of unsteady flow rate employing a centerline velocity in a circular tube. Transfer functions which relate variables in axially symmetrical laminar flow can be obtained by mathematical analysis. We applied an approximate function of transfer function between the centerline velocity and the mean flow velocity to the measurement of unsteady flow rate. An analog electronic circuit whose transfer characteristic is equivalent to the approximate transfer function can be made easily, since it is composed of a proportional element, two first order lag elements and an element based on the second order lag element. The real time measurement of unsteady flow rate can be realized with the analog electronic circuit by inputting a measured signal of the centerline velocity to it. The validity of this method is clarified experimentally by comparing it with another measuring method whose validity has been confirmed.