An ultrasonic fluid-measuring structure, comprising: a channel member having a square tube type channel in which a rectangular opening is continuous along a flow direction of a fluid; and an ultrasonic measuring section adjacent to the channel member, wherein the channel member includes a first side wall part adjacent to the ultrasonic measuring section, a second side wall part parallel to the first side wall part, a top plate part and a bottom plate part bridged between the first side wall part and the second side wall part, a first ultrasonic wave input/output section and a second ultrasonic wave input/output section provided in the first side wall part, a reflecting surface provided on an inner surface of the second side wall part, and a plurality of partition plates that are provided in parallel to the bottom plate part and divide the channel into a plurality of flat channels, the ultrasonic measuring section includes a first transmitter/receiver that transmits ultrasonic waves to the reflecting surface through the first ultrasonic wave input/output section and receives the ultrasonic waves reflected by the reflecting surface, and a second transmitter/receiver that transmits the ultrasonic waves to the reflecting surface through the second ultrasonic wave input/output section and receives the ultrasonic waves reflected by the reflecting surface, and the first ultrasonic wave input/output section and the second ultrasonic wave input/output section are continuous with each other, wherein the first side wall part comprises a rear surface side and a front surface side, said rear surface side being a channel side and said front surface side being a side opposite the channel; characterized by: an ultrasonic wave transmission membrane through which the ultrasonic waves (36, 37) pass covers both the first ultrasonic wave input/output section and the second ultrasonic wave input/output section together, wherein the ultrasonic wave transmission membrane is provided on the front surface side of the first side wall part. a channel member having a square tube type channel in which a rectangular opening is continuous along a flow direction of a fluid; and an ultrasonic measuring section adjacent to the channel member, wherein the channel member includes a first side wall part adjacent to the ultrasonic measuring section, a second side wall part parallel to the first side wall part, a top plate part and a bottom plate part bridged between the first side wall part and the second side wall part, a first ultrasonic wave input/output section and a second ultrasonic wave input/output section provided in the first side wall part, a reflecting surface provided on an inner surface of the second side wall part, and a plurality of partition plates that are provided in parallel to the bottom plate part and divide the channel into a plurality of flat channels, the ultrasonic measuring section includes a first transmitter/receiver that transmits ultrasonic waves to the reflecting surface through the first ultrasonic wave input/output section and receives the ultrasonic waves reflected by the reflecting surface, and a second transmitter/receiver that transmits the ultrasonic waves to the reflecting surface through the second ultrasonic wave input/output section and receives the ultrasonic waves reflected by the reflecting surface, and the first ultrasonic wave input/output section and the second ultrasonic wave input/output section are continuous with each other, wherein the first side wall part comprises a rear surface side and a front surface side, said rear surface side being a channel side and said front surface side being a side opposite the channel; characterized by: an ultrasonic wave transmission membrane through which the ultrasonic waves (36, 37) pass covers both the first ultrasonic wave input/output section and the second ultrasonic wave input/output section together, wherein the ultrasonic wave transmission membrane is provided on the front surface side of the first side wall part.