The relationship between the thermal efficiency and specific acoustic impedance z in a regenerator was calculated by Ceperley in 1985. His calculation results showed that regenerator thermal efficiency is limited to around 10% of the Carnot efficiency when z of the regenerator is equivalent to the characteristic acoustic impedance of a traveling wave in free space. Furthermore, he also showed that if z can become 10 times of the characteristic acoustic impedance of a traveling wave, the thermal efficiency reaches 79% of the Carnot efficiency. In order to achieve high acoustic impedance, he explained the cross sectional area of the enlarged tube should be larger than that of the upstream duct. On the other hand, losses caused by sudden expansions in regenerator was found. These kinds of losses could be reduced by installing tapered tubes between regenerator and upstream/downstream ducts. In this study, we changed the angle of the tapered tube and measured thermal efficiency and acoustic power gain at regenerator positions of a traveling wave thermoacoustic engine. From the results, maximum thermal efficiency achieved 50.6 % of Carnot efficiency when taper angle was 7 degree.