An ultrasonic flow rate measuring device comprising: a rectangular flow path in which a target fluid flows, the flow path including: a first sidewall that is provided along a flow direction of the target fluid, a second sidewall that is disposed facing the first sidewall, a bottom plate that extends from a bottom of the first sidewall to a bottom of the second sidewall, an upper wall unit that connects an upper portion of the first sidewall and an upper portion of the second sidewall, and an aperture that is provided in the upper wall unit facing the bottom plate; a partition plate that is inserted from the aperture to partition the flow path into a plurality of sections; an ultrasound transmission body that covers the aperture; a plurality of ultrasonic transducers (16, 17) that are provided in positions facing the bottom plate such that an ultrasonic wave transmitted from one of the ultrasonic transducers (16, 17) through the ultrasound transmission body is reflected by the bottom plate and received by the other ultrasonic transducer (16, 17); a measurement circuit that measures an ultrasonic propagation time between the plurality of the ultrasonic transducers (16, 17); and a calculation circuit that obtains a flow rate of the target fluid based on a signal from the measurement circuit; wherein the partition plate includes a plurality of protrusions (11, 12) that protrude in both side ends and a lower end, and the upper wall unit and the bottom plate include insertion grooves (13, 14) in which the protrusions (11, 12) are inserted; characterized in that: an upper portion of the insertion groove of the bottom plate is greater than a width of the protrusion and a lower portion of the insertion groove is less than the width of the protrusion. a rectangular flow path in which a target fluid flows, the flow path including: a first sidewall that is provided along a flow direction of the target fluid, a second sidewall that is disposed facing the first sidewall, a bottom plate that extends from a bottom of the first sidewall to a bottom of the second sidewall, an upper wall unit that connects an upper portion of the first sidewall and an upper portion of the second sidewall, and an aperture that is provided in the upper wall unit facing the bottom plate; a first sidewall that is provided along a flow direction of the target fluid, a second sidewall that is disposed facing the first sidewall, a bottom plate that extends from a bottom of the first sidewall to a bottom of the second sidewall, an upper wall unit that connects an upper portion of the first sidewall and an upper portion of the second sidewall, and an aperture that is provided in the upper wall unit facing the bottom plate; a partition plate that is inserted from the aperture to partition the flow path into a plurality of sections; an ultrasound transmission body that covers the aperture; a plurality of ultrasonic transducers (16, 17) that are provided in positions facing the bottom plate such that an ultrasonic wave transmitted from one of the ultrasonic transducers (16, 17) through the ultrasound transmission body is reflected by the bottom plate and received by the other ultrasonic transducer (16, 17); a measurement circuit that measures an ultrasonic propagation time between the plurality of the ultrasonic transducers (16, 17); and a calculation circuit that obtains a flow rate of the target fluid based on a signal from the measurement circuit; wherein the partition plate includes a plurality of protrusions (11, 12) that protrude in both side ends and a lower end, and the upper wall unit and the bottom plate include insertion grooves (13, 14) in which the protrusions (11, 12) are inserted; characterized in that: an upper portion of the insertion groove of the bottom plate is greater than a width of the protrusion and a lower portion of the insertion groove is less than the width of the protrusion. an upper portion of the insertion groove of the bottom plate is greater than a width of the protrusion and a lower portion of the insertion groove is less than the width of the protrusion.