The object is to facilitate determination of a flow rate of fluid discharged from a discharge opening.Intensity of a supersonic wave generated at and propagated from a discharge opening in association with discharge of the fluid from the discharge opening is determined at a determinate site distant from the discharge opening. And, a propagation distance from the discharge opening to the determination site is determined or investigated. Then, based upon a correlation existent among the intensity of the propagated supersonic wave, the propagation distance and the fluid discharge flow rate from the discharge opening, the fluid discharge rate is obtained from the determined or investigated supersonic wave intensity and the propagation distance.