PURPOSE: To obtain the titled composition having increased safety and sustaining the generation of Cl2 and/or ClO2 for a long term by mixing a thixotropic composition contg. polysaccharide with a Cl2 and/or ClO2 generating agent.

CONSTITUTION: To a thixotropic composition contg. polysaccharide is added 0.05W0.5wt% amphoteric surfactant such as alkylbetaine, and a Cl2 and/or ClO2 generating agent is further added so that the concn. of available chlorine in the resulting composition becomes 0.5W10wt%, preferably about 1W7wt%. The resulting composition generates stably gaseous Cl2 and/or ClO2, it can generate a very small amount of gaseous Cl2 and/or ClO2 over a long term, and the superior stability is not affected by a sudden change in the temp. of the environment. The polysaccharide can be selected from various kinds of polysaccharides including homo- and heteropolysaccharides.