An ultrasonic flow-meter comprising: a pair of ultrasonic transducers (7, 8) which are disposed in a measurement passage; a measuring unit which measures a propagation time of an ultrasonic wave between the pair of ultrasonic transducers (7, 8); and a calculating unit which calculates a flow rate of a target fluid, characterized in that: the ultrasonic flow meter further comprises only one partition plate (3; 20) which partitions a passage of the target fluid into the measurement passage and a non-measurement passage; wherein the calculating unit has a computing unit which computes at least one of a flow velocity and a flow rate (Qm) 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 whole passage based on the flow velocity or the flow rate (Qm) both in the measurement passage; wherein from the flow velocity multiplications by the measurement passage cross section S and by a correction factor p are necessary to obtain the flow rate (Qm) in the measurement passage, from the said flow rate (Qm) a multiplication by a factor q is necessary to obtain the flow rate in the whole passage, and the ultrasonic flow meter further comprising a structural body which makes the target fluid become turbulent in both the measurement passage and the non-measurement passage at the same time, wherein the structural body is a resistance body disposed in the non-measurement passage. a pair of ultrasonic transducers (7, 8) which are disposed in a measurement passage; a measuring unit which measures a propagation time of an ultrasonic wave between the pair of ultrasonic transducers (7, 8); and a calculating unit which calculates a flow rate of a target fluid, characterized in that: the ultrasonic flow meter further comprises only one partition plate (3; 20) which partitions a passage of the target fluid into the measurement passage and a non-measurement passage; wherein the calculating unit has a computing unit which computes at least one of a flow velocity and a flow rate (Qm) 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 whole passage based on the flow velocity or the flow rate (Qm) both in the measurement passage; wherein from the flow velocity multiplications by the measurement passage cross section S and by a correction factor p are necessary to obtain the flow rate (Qm) in the measurement passage, from the said flow rate (Qm) a multiplication by a factor q is necessary to obtain the flow rate in the whole passage, and the ultrasonic flow meter further comprising a structural body which makes the target fluid become turbulent in both the measurement passage and the non-measurement passage at the same time, wherein the structural body is a resistance body disposed in the non-measurement passage. the ultrasonic flow meter further comprises only one partition plate (3; 20) which partitions a passage of the target fluid into the measurement passage and a non-measurement passage; wherein the calculating unit has a computing unit which computes at least one of a flow velocity and a flow rate (Qm) 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 whole passage based on the flow velocity or the flow rate (Qm) both in the measurement passage; wherein from the flow velocity multiplications by the measurement passage cross section S and by a correction factor p are necessary to obtain the flow rate (Qm) in the measurement passage, from the said flow rate (Qm) a multiplication by a factor q is necessary to obtain the flow rate in the whole passage, and the ultrasonic flow meter further comprising a structural body which makes the target fluid become turbulent in both the measurement passage and the non-measurement passage at the same time, wherein the structural body is a resistance body disposed in the non-measurement passage.