The production process of high purity fused silica for package fillers of semiconductor elements was developed. In the silica melting process via flame combustion. a large-sized furnace made of the partially stabilized zirconia bricks equipped with a compact and highly efficient packed bed preheater was developed to increase the productivity and thermal efficiency. A continuous fusing furnace, in which the silica ingot is drawn from the furnace bottom, was developed. Since the silica was not contaminated in the furnace, the fused silica was produced in high purity. As the ingot was quenched quickly, it was of high quality with a low cristobalite ratio.