To obtain a method and an apparatus in which the speed of a fluid can be computed with high reliability and with high accuracy by a method wherein, when the phase difference between a signal which is inphase and a signal which is at a quadrature phase is neither +90° nor -90° anticipating a prescribed error value, their spectral components are regarded as noises so as to be removed before an average flow velocity is computed.

In the Doppler shift which is generated in a fluid 7 flowing toward a transducer 2, for reception, at a sensor probe 3 arranged in the bottom part of a flow passage 4, the phase of a signal which is inphase is by +90° different from that of a signal which is at a quadrature phase. On the other hand, in the case of a fluid which flows to its opposite direction, it is confirmed that the phase of the signal which is inphase is by -90° different from that of the signal which is at the quadrature phase. By this invention, even when the phase difference between the signal which is inphase and the signal which is at the quadrature phase is neither +90° nor -90° anticipating a prescribed error value, their spectral components are regarded as noises, they are weighted by 0, and they are removed before a later computing operation. Consequently, an average flow velocity which is found by the computing operation has a high-reliability and high-accuracy value which does not contain any noise components.