PURPOSE: To reduce the utilization of heat of reaction of expensive SO3 and to enable advantageous selection of a material for an apparatus for reacting fluorite with sulfuric acid in the 1st and the 2nd reactors by substituting SO3 and steam for part of sulfuric acid in the 1st reactor and restricting the reaction temp.

CONSTITUTION: Fluorite is first reacted with SO3 and H2O in the 1st reactor, and almost simultaneously or a little later remaining sulfuric acid necessary for the reaction of fluorite is fed as one or more among sulfuric acid, fuming sulfuric acid and fluorosulfuric acid recovered in an HF refining stage. The reaction temp. is regulated to 140W240°C, and the reaction is allowed to proceed until about 35W90% rate is attained. The reaction product is a nonsticky solid. It is fed to the 2nd reactor and further reacted after supplying a shortage of sulfuric acid as required. At this time, heat of combustion of heavy oil, etc. can be utilized besides heat of reaction of expensive SO3 with H2O, and the reaction is completed in a short time with high thermal efficiency in compact equipment without forming a viscous solid product.