To provide a method of making chitin and chitosan low-molecular, which can make chitin and chitosan low-molecular in a short time and without needing a complicated post-treatment, and also can produce the decomposition products in large quantities to reduce costs, the decomposition products being utilized as foodstuffs.

Amorphous chitin or amorphous chitosan is decomposed under a hydrothermal condition of 130-220°C to make the amorphous chitin or the amorphous chitosan low-molecular. A sealing vessel, into which a fluid dispersion formed by dispersing amorphous chitin or amorphous chitosan in water is introduced and which is sealed so as to remove freely the contents, is accomodated in a high-pressure steam kettle to perform the decomposition. The decomposition is carried out in the presence of an acid catalyst in an amount five molar times or less that of the amorphous chitin or the amorphous chitosan. The decomposition is controlled by the reaction time and/or the reaction temperature.