The purpose of this study is to reduce cavitation noise generated behind a butterfly-valve set in a straight pipe. The main reason of cavitation occurrence is an abrupt decrease of the pressure behind the valve. In our experiments, the duct was partly enlarged to recover the pressure behind the valve and to reduce cavitation occurrence. It was confirmed that this method is effective to reduce cavitation through the experiments using a pipe with a circular cross section. Moreover, we carried out the experiments using butterfly-valve with aileron. And we confirmed that the effect of the aileron on the noise was not so large.