In order to improve the accuracy of the measurement of cylinder diameter by means of phase delay of sound waves, the effects of echoes from a pair of rigid walls have been studied. The cylinder of interest was located midway between the walls which were parallel to the impinged sound waves. Experiments having been made using a loudspeaker, microphones and a FFT-analyzer, and the problem of the disturbance of plane waves by a cylinder having been analyzed theoretically, relation-ships between the phase delay and the diameter were obtained, respectively. The results showed that the rate of increase of the phase delay with diameter, which was considerd approximately constant before utilizing echoes, became much larger within a certain diameter range which was dependent on the frequency of sound.