PURPOSE: To provide a blood flowmeter obtaining a stable result while reflecting a change in blood flow, by providing a setting means capable of arbitrarily setting the number of data of the optimum moving average corresponding to blood flow change velocity.

CONSTITUTION: The laser beam emitted from a laser beam source 1 passes through a beam transmitting fiber 2 to irradiate a specimen S from the leading end of a probe 3. A part of beam scattered by the blood flow in the specimen S and containing the data of blood flow is returned to a probe 3 to be incident to a photoelectric detector 5 through a beam receiving fiber 4 and converted to an electric signal which is, in turn, amplified by an amplifier 6 to be separated into high and low frequency components by two kinds of band pass filters 7, 8 and these two peak values are converted to digital quantities by an AD converter 9 to be inputted to an operator 10 and instantaneously blood flow velocity is obtained. In the operator 10, the contact of an external switch 12 is detected to input the number of data when moving average is taken and this number data is used to output the blood flow value calculated by moving average to a display part 11.