Abstract: A high strength rock wool obtained by preliminarily calcining the starting material comprising industrial wastes such as shot and the like generated in the course of the production of a rock wool in a calcination temperature region of from 1,000.degree. to 1,250.degree. C. for a time of at least 1 minute but within 60 minutes in an oxidizing atmosphere, immediately crushing and pulverizing the resulting block-shaped calcined product, introducing the resulting particle-shaped calcined product into an electric furnace to melt the same and fiberizing the resulting melt by a spinner wheels and a process for producing the same. The said rock wool has a high strength appropriate to utilize in a thermal insulation material, a sound absorbing material and various reinforcing fibers.