An ultrasonic flow meter unit comprising: a measurement fluid passage through which a fluid flows; a pair of ultrasonic transducers (10a, 10b) provided on the measurement fluid passage; a measuring circuit board including a measuring section for measuring a propagation time of an ultrasonic signal transmitted and received between the ultrasonic transducers (10a, 10b), and a fluid flow calculating section for calculating a fluid flow based on a signal from the measuring section; a board holder for holding the measuring circuit board; characterised in that: lead pins are connected to terminals of the ultrasonic transducers (10a, 10b); the pair of ultrasonic transducers (10a, 10b) are mounted to the measurement fluid passage, the board holder holding the measuring circuit board is mounted to the measurement fluid passage, and then the lead pins are connected to the measuring circuit board, one end of each of the lead pins is connected to a surface of the pair of ultrasonic transducers (10a, 10b) which is on an opposite side of a transmission/reception surface of the pair of ultrasonic transducers (10a, 10b) before the pair of ultrasonic transducers (10a, 10b) are mounted to the measurement fluid passage, and the lead pins are inserted into terminal holes of the measuring circuit board and solder-mounted thereto after the pair of ultrasonic transducers (10a, 10b) are mounted to the measurement fluid passage. a measurement fluid passage through which a fluid flows; a pair of ultrasonic transducers (10a, 10b) provided on the measurement fluid passage; a measuring circuit board including a measuring section for measuring a propagation time of an ultrasonic signal transmitted and received between the ultrasonic transducers (10a, 10b), and a fluid flow calculating section for calculating a fluid flow based on a signal from the measuring section; a board holder for holding the measuring circuit board; characterised in that: lead pins are connected to terminals of the ultrasonic transducers (10a, 10b); the pair of ultrasonic transducers (10a, 10b) are mounted to the measurement fluid passage, the board holder holding the measuring circuit board is mounted to the measurement fluid passage, and then the lead pins are connected to the measuring circuit board, one end of each of the lead pins is connected to a surface of the pair of ultrasonic transducers (10a, 10b) which is on an opposite side of a transmission/reception surface of the pair of ultrasonic transducers (10a, 10b) before the pair of ultrasonic transducers (10a, 10b) are mounted to the measurement fluid passage, and the lead pins are inserted into terminal holes of the measuring circuit board and solder-mounted thereto after the pair of ultrasonic transducers (10a, 10b) are mounted to the measurement fluid passage. lead pins are connected to terminals of the ultrasonic transducers (10a, 10b); the pair of ultrasonic transducers (10a, 10b) are mounted to the measurement fluid passage, the board holder holding the measuring circuit board is mounted to the measurement fluid passage, and then the lead pins are connected to the measuring circuit board, one end of each of the lead pins is connected to a surface of the pair of ultrasonic transducers (10a, 10b) which is on an opposite side of a transmission/reception surface of the pair of ultrasonic transducers (10a, 10b) before the pair of ultrasonic transducers (10a, 10b) are mounted to the measurement fluid passage, and the lead pins are inserted into terminal holes of the measuring circuit board and solder-mounted thereto after the pair of ultrasonic transducers (10a, 10b) are mounted to the measurement fluid passage.