A flow-rate measuring device comprises a cavity and an inlet port for receiving and introducing a flow of fluid to the cavity. A wobble plate is pivotally disposed in the cavity to equally divide it into first and second cavity sections when in equilibrium position. An arcuate baffle plate is disposed in the cavity. First and second discharge channels are formed between the edges of the baffle plate and the inner wall of the cavity. An outlet port is connected to the discharge channels. The rearward end of the wobble plate is in proximity to the arcuate surface of the baffle plate and the pivot point of the wobble plate lies at the center of curvature of the baffle plate. The cavity has an increasing transverse dimension toward the first and second discharge channels to form a converging space toward each discharge channel when the wobble plate takes an extremely inclined position.