We proposed a new technique for stirring polymer melt in a mold by acoustic streaming to prevent formation of weldlines in injection molding process. We have demonstrated the generation of the acoustic streaming in high viscosity polymer melt. We radiated ultrasound in a disk-shaped mold to induce the acoustic streaming in the thickness direction of the mold. The acoustic streaming in the polymer melt was examined by observing, after cooling, the displacements of tracers in the cross section of the molded product. We have succeeded in generating the acoustic streaming in the polymer melt of 11000Pa·s in the mold. The acoustic streaming was, however, strongly affected by reflected waves from the surface of the melt. We have found that the acoustic streaming can be efficiently generated in the mold if the interference with the reflected waves are decreased by using a tone-burst ultrasound.