An ultrasonic flow meter including a straight tube-shaped tubular body for measurement through which a fluid for measuring a flow rate flows has been conventionally known (for example, see Japanese Unexamined Patent Application, Publication No.  2012-42243 (hereinafter referred to as “JP 2012-42243”)). 
The ultrasonic flow meter disclosed in JP 2012-42243 has a structure in which a pair of ultrasonic wave oscillators is disposed at a predetermined interval on the outer periphery of the straight tube-shaped tubular body for measurement.