PURPOSE: To permit stable addition of a water-treating agent, permitting also easy inspection of remaining amt. of the agent by packing a solid water-treating agent in a bag provided with water permeable holes being made of a combination of nonwoven cloth and flexible metallic foil or plastic film.

CONSTITUTION: A part of the inside of a bag 1 made of nonwoven cloth 11 having water permeability is covered with a film material 9 having no water permeability, or the inside surface of the bag 1 is covered with the film material 9 having many water permeating holes 10. A solid water treating agent 5 is packed in the bag. Aimed effect is obtd. by dissolving the agent with a specified dissolving velocity by allowing the agent 5 to contact securely with water by floating the bag or settling at a specified position in the water. Stable addition of the agent at intervals of a fixed time is also possible. Inspection of remaining amt. of the agent is easy. Further, several modifications are also possible, so satisfactory effect in the water treatment of several scale, particularly in the middle and small scale water treatment may be achieved.