An ultrasonic flow-meter of the present invention includes a partition plate which partitions a passage of a target fluid into a measurement passage and a non-measurement passage, a pair of ultrasonic transducers which is disposed in the measurement passage, a measuring unit which measures a propagation time of an ultrasonic wave between the pair of ultrasonic transducers, and a calculating unit which calculates a flow rate of the target fluid. Further, the calculating unit has a computing unit which computes at least one of a flow velocity and a flow rate of the target fluid in the measurement passage based on the propagation time and an estimating unit which estimates the flow rate of the target fluid in the passage based on the flow velocity or the flow rate in the measurement passage. As a result, an ultrasonic flow-meter which highly accurately measures a target fluid by using a simple configuration can be realized.