PURPOSE: To provide the titled carbon material having excellent heat-insulation and compressive strength and high dimensional stability, by heat-treating the carbon material in the calcination process at a specific temperature for a specific time interval or longer.

CONSTITUTION: Raw materials such as coke, pitch, coal tar, etc. are kneaded, crushed, mixed and formed. The formed raw material is calcined at ≤about 1,000°C and heat-treated at 1,100W1,300°C for at least 50hr, preferably >50hr. The heat-treatment may be carried out without prior calcination. The obtained carbon material has excellent dimensional stability and heat-insulation at the working temperature as the heat-insulator of the floor of a high temperature gas-cooled reactor such as helium gas cooling nucleus reactor, etc.