PROBLEM TO BE SOLVED: To provide a solid water treatment agent excellent in long term preservation, having sustained release in which an active ingredient is discharged into a water system gradually and stably with time in use.SOLUTION: A solid water treatment agent having sustained release is constituted as follows. In the water treatment agent, a water-soluble water treatment medicine exists in a copolymer as a solid solution or a dispersoid. The copolymer is a copolymer composed of three components, namely, N-vinyl caprolactam, vinyl acetate and polyethylene glycol.
