An ultrasonic flowmeter that measures flow velocity of a measured fluid by propagating ultrasonic waves through a measuring flow path with a rectangular cross section through which the measured fluid flows and the measured fluid that flows through the measuring flow path. The measuring flow path is provided with partition plates so that they are in parallel with the flow direction of the measured fluid. Simultaneously, the partition plates are placed in parallel with wall surfaces such that flow velocity distribution between opposing wall surfaces of the measuring flow path is more symmetrical with respect to the center of the flow direction of the measured fluid.