A disposable conduit 1 includes piezo-electric elements 2a and 2b and a memory chip 10 in which calibration data specific to the relevant conduit is stored. Before measurement, a non-disposable electrical measuring circuit reads the calibration data out of the memory chip 10 to prepare a calibrating equation or a calibration table. A flow speed of a blood passing through the conduit or a flow rate calculated from the flow speed is corrected in accordance with the calibrating equation or calibration table.