An ultrasonic flowmeter comprising: a flow path that has a rectangular cross section and through which a fluid to be measured flows; a partition plate that divides the flow path into a plurality of layers to form layered flow paths (12, 13a, 13b); a pair of ultrasonic sensors (2a, 2b) that are arranged upstream and downstream of a surface forming the flow path and transmitting and receiving ultrasonic signals; a flow rate calculator configured to detect a flow rate of the fluid to be measured based on a propagation time from when the ultrasonic signals are transmitted from one of the pair of ultrasonic sensors (2a, 2b) until when the ultrasonic signals are received by another one of the pair of ultrasonic sensors (2a, 2b) after propagating through the fluid to be measured; mounting portions (3a, 3b) that mount the ultrasonic sensors (2a, 2b) on the flow path; and openings that are provided directly below the mounting portions (3a, 3b) and through which ultrasonic waves pass, each of the openings is open to face a plurality of layers of the layered flow paths (12, 13a, 13b), characterised in that the plurality of layered flow paths (12, 13a, 13b) includes a measurement flow path for detecting the flow rate of the measured fluid and non-measurement flow paths (13a, 13b) other than the measurement flow path, each of the openings comprises a smaller opening (14b, 14e) facing the measurement flow path and smaller openings (14a, 14c, 14d, 14f) each facing a corresponding one of the non-measurement flow paths (13a, 13b) and that the ultrasonic sensors (2a, 2b) are configured to propagate the ultrasonic waves mainly to the measurement flow path. a flow path that has a rectangular cross section and through which a fluid to be measured flows; a partition plate that divides the flow path into a plurality of layers to form layered flow paths (12, 13a, 13b); a pair of ultrasonic sensors (2a, 2b) that are arranged upstream and downstream of a surface forming the flow path and transmitting and receiving ultrasonic signals; a flow rate calculator configured to detect a flow rate of the fluid to be measured based on a propagation time from when the ultrasonic signals are transmitted from one of the pair of ultrasonic sensors (2a, 2b) until when the ultrasonic signals are received by another one of the pair of ultrasonic sensors (2a, 2b) after propagating through the fluid to be measured; mounting portions (3a, 3b) that mount the ultrasonic sensors (2a, 2b) on the flow path; and openings that are provided directly below the mounting portions (3a, 3b) and through which ultrasonic waves pass, each of the openings is open to face a plurality of layers of the layered flow paths (12, 13a, 13b), characterised in that the plurality of layered flow paths (12, 13a, 13b) includes a measurement flow path for detecting the flow rate of the measured fluid and non-measurement flow paths (13a, 13b) other than the measurement flow path, each of the openings comprises a smaller opening (14b, 14e) facing the measurement flow path and smaller openings (14a, 14c, 14d, 14f) each facing a corresponding one of the non-measurement flow paths (13a, 13b) and that the ultrasonic sensors (2a, 2b) are configured to propagate the ultrasonic waves mainly to the measurement flow path.