To provide a fluid flow rate detection device which can be evaluated as adequate from a medical point of view. The velocity of a fluid flowing through a luminal organ in vivo is to be obtained. Vθ (r, θ)=w•V-θ+(1-w)•V+θ is calculated to obtain a calculated value Vθ(r, θ) of the flow rate regarding a component of the fluid in the direction perpendicular to the ultrasonic beam direction. Here, the weight w is a value proportional to the distance d from the wall on one side of the organ at least when the distance is smaller than a predetermined distance from the wall on the one side, and the weight is a value proportional to the distance d from the wall on the other side of the organ at least when the distance is smaller than a predetermined distance from the wall on the one other side.
