After adding water to the carboxymethyl carrageenan which is raw material and mixing them well, the ionizing radiation of more than fixed dose is irradiated to the obtained paste sample of fixed concentration. As a result, an excellent hydrogel in heat resistance which does not dissolve at 50.degree. C. or more can be obtained. The manufactured gel can be used for many kinds of products. Because this gel has a biodegradation characteristic, it is possible to dispose by composting.