PURPOSE: To form a chitosan film to the surface of a granule in a uniform state, by preliminarily adding Na-polyphosphate to a core substance to form a granular substance which is, in turn, added to a dilute aqueous chitosan solution under stirring.

CONSTITUTION: Na-polyphosphate is preliminarily added to a core substance (e.g., theophylline) to form a granular substance which is, in turn, added to a dilute aqueous chitosan solution under stirring to form a chitosan film to the surface of the granular substance in a uniform state. By changing the concn. of the chitosan solution, the thickness of the film covering the core substance or the hardness of the film can be changed. On the other hand, from the aspect of the concn. of the chitosan solution, and the quality and the operability of the film to be coated, for example, from the aspect of the quality of the coating state to the core substance in drugs or processed foods, the optimum concn. of chitosan resides in the vicinity of about 0.3W0.9%.