Abstract: A clamp-on ultrasonic flowmeter includes: a first ultrasonic transducer that injects a first ultrasonic signal at an angle in excess of a critical angle into a pipe wherein a fluid flows; a second ultrasonic transducer, provided in a position able to receive the first ultrasonic signal, which injects a second ultrasonic signal at the same angle as the aforementioned angle, relative to the pipe; and a flow rate calculating portion that calculates either one of or both of a flow speed and a flow rate of the fluid within the pipe based on a first time for the first ultrasonic signal to pass through the interior of the pipe to arrive at the second ultrasonic transducer and a second time for the second ultrasonic signal to pass through the interior of the pipe to arrive at the first ultrasonic transducer.