In measurement of an ultrasonic flowmeter, the flow rate in piping is obtained using some measurement signals along the "chord" of the circular cross section of the piping. As for the "Chordal type ultrasonic flowmeter", the flow rate is obtained by four (or eight) "chords". The wall roughness influences the velocity profile. To evaluate the influences of roughness change on the measurement using four velocity points, a water test was performed. In the wall where sand of 0.1mm diameter was pasted, no difference in the velocity profile with the smooth wall was observed. A change in the velocity profiles appeared in the rough surface pipe where sands of 0.6mm and 0.2mm diameter has been pasted. It was suggested that the change could be diagnosed by the surveillance parameter VSLR when the inside surface of pipe roughness changed. It was found that the Reynolds number dependency of the measurement and the sensitivity of the roughness are small.