In-line defoaming of highly viscous fluids is one of the remaining challenges that needs to be addressed for realizing their industrial use. To overcome the problem of air bubbles in such fluids, the pressure-oscillating defoaming for shear-thinning fluids has been developed. This method is performed by intentionally applying pressure oscillation to a fluid containing a bubble to produce a continuous and strong local flow around the alternately contracting/expanding bubble, where the shear viscosity has been lowered. On the basis of the success that we achieved using this technique for causing bubbles in a quartz cell to rise faster, we applied it to a branched-flow channel filled with a viscoelastic fluid with a zero-shear viscosity of 90 Pa.s for the purpose of developing an in-line defoaming system. In this study, the motions of bubbles in a revised flow channel filled with a higher-viscosity liquid (220 Pa.s) were investigated experimentally. In addition, a small air cavity was introduced on the upper side of the flow channel to act as a bubble trap for improving the defoaming performance. We studied the effect of the trap position on the defoaming performance.