An ultrasonic flowmeter comprising: a measurement flow path through which a fluid to be measured flows, the measurement flow path including a first surface, a second surface facing the first surface, and a third surface and a fourth surface laid between the first surface and the second surface to form a rectangular cross section, said measurement flow path having a center line; a pair of ultrasonic sensors (2a, 2b) that are arranged upstream and downstream on the first surface of the measurement flow path and capable of transmitting and receiving ultrasonic signals, wherein the pair of ultrasonic sensors (2a, 2b) are fixed to the measurement flow path, incident angles (θ) of ultrasonic signals transmitted from the pair of the ultrasonic sensors (2a, 2b) on a reflection surface of the second surface where the ultrasonic signals are reflected being equal, openings (6a, 6b) that are provided on the first surface for the pair of ultrasonic sensors (2a, 2b) to transmit and receive the ultrasonic signals; and a flow rate calculator that detects a flow rate of the fluid to be measured based on a propagation time until ultrasonic signals transmitted from one of the pair of ultrasonic sensors (2a, 2b) propagate through the fluid to be measured with at least one reflection on the second surface of the measurement flow path, and are received by another one of the pair of ultrasonic sensors (2a, 2b), characterized in that: the measurement flow path has an inner wall surface (4, 5, 15, 16) provided with drafts, the drafts are provided on the first surface and the second surface, each of the first surface and the second surface have drafts on both a side of a flow path inlet and on a side of a flow path outlet, an angle of the draft of the first surface with respect to the center line on the side of the flow path inlet is the same as an angle of the draft of the first surface with respect to the center line on the side of the flow path outlet, and the reflection surface provided on the second surface does not have a draft. a measurement flow path through which a fluid to be measured flows, the measurement flow path including a first surface, a second surface facing the first surface, and a third surface and a fourth surface laid between the first surface and the second surface to form a rectangular cross section, said measurement flow path having a center line; a pair of ultrasonic sensors (2a, 2b) that are arranged upstream and downstream on the first surface of the measurement flow path and capable of transmitting and receiving ultrasonic signals, wherein the pair of ultrasonic sensors (2a, 2b) are fixed to the measurement flow path, incident angles (θ) of ultrasonic signals transmitted from the pair of the ultrasonic sensors (2a, 2b) on a reflection surface of the second surface where the ultrasonic signals are reflected being equal, openings (6a, 6b) that are provided on the first surface for the pair of ultrasonic sensors (2a, 2b) to transmit and receive the ultrasonic signals; and a flow rate calculator that detects a flow rate of the fluid to be measured based on a propagation time until ultrasonic signals transmitted from one of the pair of ultrasonic sensors (2a, 2b) propagate through the fluid to be measured with at least one reflection on the second surface of the measurement flow path, and are received by another one of the pair of ultrasonic sensors (2a, 2b), the measurement flow path has an inner wall surface (4, 5, 15, 16) provided with drafts, the drafts are provided on the first surface and the second surface, each of the first surface and the second surface have drafts on both a side of a flow path inlet and on a side of a flow path outlet, an angle of the draft of the first surface with respect to the center line on the side of the flow path inlet is the same as an angle of the draft of the first surface with respect to the center line on the side of the flow path outlet, and the reflection surface provided on the second surface does not have a draft.