In a secondary beam method, scanning is performed using Doppler measurement ultrasound beams as primary beams, and Doppler measurement components are measured on the basis of the ultrasound waves received from the direction of each Doppler measurement ultrasound beam. Then, ultrasound waves forming secondary beams are sent and received, and the Doppler effect for the secondary beam direction component at the point of intersection of the Doppler measurement ultrasound beam with the secondary beams is measured. Furthermore, using the intersection point as the position for starting integration, an integration calculation based on the law of conservation of mass is performed along the route that intersects with the Doppler measurement ultrasound beams, and the component in the direction of the intersection route is found. The initial value for integration is found on the basis of the Doppler measurement component and the secondary beam direction component at the integration start point.
