PURPOSE: To reduce degassing at a high temperature and to obtain high insulation by decreasing heat loss at the time of melt reacting high melting point substance having a predetermined high temperature as its melting point, and forming a radiation shielding material for obtaining high temperature by thermally decomposed boron nitride.

CONSTITUTION: When substance having a melting point of 2000°C or higher such as Ce pnictide or Yb pnictide is single crystallized or reacted in a solution tank, a heat shielding material is used to reduce heat loss due to radiation heat of the substance and to obtain a desired temperature. As the material, thermally decomposed boron nitride is used, and heat insulating radiation absorption layer of thermally decomposed carbon, molybdenum, titanium is provided on a surface of the nitride as required. The nitride is generated by heating halogenated boron and ammonia in vacuum to thermally decompose the boron, reacting it with the ammonia and depositing the obtained boron nitride on a carrier.