PURPOSE: To form a shell to a gel sphere and to simultaneously form a hollow part in the sphere by coating the surface of the core material of the gel sphere formed from an aqueous polysaccharide solution with a powder of a shell forming material and dehydrating the gel sphere by heat treatment.

CONSTITUTION: At first, an aqueous solution of polysaccharide such as alginic acid or sodium alginate is dispersed in a gelling solution composed of an aqueous solution of calcium chloride or silver chloride to form a gel sphere becoming a core material. Next, the surface of the core material is coated with a powder of a shell forming material, for example, various ceramics, cement or a metal material such as iron. At this time, the moisture in the gel sphere transfers to the surface thereof to be absorbed in the powder layer. Finally, the gel sphere is heat-treated to dehydrate the powder layer containing the moisture absorbed from the gel sphere and, at the same time, a hollow part is formed in a shell. By this method, a light hollow sphere having high strength and excellent in heat insulating properties can be prepared in an extremely easy manner.