An ultrasonic flowmeter includes a flow path defined by four wall sections, a flow measurement section, a calculation section and a pair of ultrasonic oscillators for transmitting and receiving an ultrasonic wave over a distance. The flow path is configured to reduce the influence of a phase difference, which results from the varying propagation distances of a direct wave and a reflected wave, by changing one or more of the flow measurement section configuration, the ultrasonic wave frequency, the orientation of the ultrasonic oscillators, and the distance between the pair of ultrasonic oscillators. A measurement section is suitably configured to measure the propagation time of the ultrasonic wave propagating between the pair of ultrasonic oscillators. Additionally, a calculation section is suitably configured to receive measurements from the measurement section and to calculate the fluid flow in the flow measurement section.