To provide an ultrasonic flowmeter for measuring the flow volume of a measuring fluid based only on a flow velocity in a tube axis direction by removing flow velocity components such as drift included in flow velocity components of the measuring fluid measured by first transmission/reception ultrasonic vibrators by being equipped with second transmission/reception ultrasonic vibrators disposed at positions confronting a measurement tube in its tube cross-section direction in addition to the first ultrasonic vibrators disposed with prescribed intervals kept in the tube axis direction of the measurement tube.

This ultrasonic flowmeter for measuring the flow volume of the measuring fluid flowing through the measurement tube by using an ultrasonic wave, is equipped with the plurality of first transmission/reception ultrasonic vibrators disposed with prescribed intervals kept in the tube axis direction of the measurement tube, and the plurality of second transmission/reception ultrasonic vibrators disposed at positions confronting the measurement tube in its tube cross-section direction. The flow volume of the measuring fluid is calculated by removing the flow velocity components measured by the second ultrasonic vibrators from the flow velocity components measured by the first ultrasonic vibrators.