Abstract: A granular carbonaceous heat insulator comprising a carbon black pellet the surface of which is coated with a carbon layer having a dense structure is disclosed. This heat insulator is prepared by adhering a binder solution to a carbon black pellet, heating the resulting coating to remove a solvent from the binder solution and, at the same time, to cure the binder, and baking the carbon black pellet coated with the cured binder in a non-oxidizing atmosphere to carbonize the cured binder. Alternatively, the heat insulator is prepared by subjecting a carbon black pellet having specific characteristics to wet granulation in the presence of a binder solution to crush the carbon black pellet and, at the same time, to granulate the crushed pellet, thereby preparing a granulated carbon black pellet, followed by removal of the solvent from the prepared pellet, curing of the binder and baking of the cured binder in the same manner as that described above.