In this study, we focused on the fiber yielding system from fused coal ash to expand the ash utilization as building materials because slag inhibits the elution of toxic heavy metals, such as Hg, Pb, and As, to undetectable levels. In particular, fiber materials from coal ash slag are expected to be utilized as thermal insulator and acoustic materials. We designed a coal ash fusing furnace which is suitable for fiber manufacturing and recovery processes. To manufacture safe and homogeneous fibrous materials, slag viscosity had to be stabilized as approximately 1Pa・s at the slag-tapping hole of the furnace. But the slag temperature at 1Pa・s was too high to measure. We estimated the temperature at 1Pa・s using Riboud's and Urbain's expression. To control the slag viscosity and temperature, CaCO_3 addition to coal ash was effective. When the basicity (CaO/SiO_2 (wt%/wt%)) of the coal ash sample was 0.98, the fused slag temperature at 1Pa・s was estimated as 1563℃. Then, we verified that to yield 1Pa・s fusing slag from 120kg/h coal ash, the necessary fuel quantity of the coal ash fusing furnace was 40kg/h coal and 4m^3N/h LPG.