PURPOSE: To raise water-stopping effect of concrete joints by a method in which a swelling water-stopping agent is buried in the joint between an existing con crete and fresh concrete or in chipped portion and swelled for gelation,

CONSTITUTION: A swelling water-stopping agent 1 is buried in the joint 5 between an existing concrete structure 2 already cured and a connecting fresh concrete 4 placed on the concrete structure 2. The agent 1 is formed by packing inorganic water-absorbing dried gel grains 10 and 10a such as bentonite into a small and long meshed bag 11. The agent 1 absorbs water from the concrete 4 and water coming from the apertures 14 after the concrete 4 is placed and swells to gelling. The aperture 14 of the joint 5 is automatically closed by the swelled gel. Semipermanent water-stopping function can thus be maintained to eliminate the needs for periodic water-proofing and repairing operations.