Provided is a water-based patch that contains ketoprofen as an active ingredient, provides high transdermal absorbability of the ketoprofen, and has high safety and high storage stability. The water-based patch contains the ketoprofen, an amine, and polyethylene glycol. In the water-based patch, the added amount of the ketoprofen in a paste is 0.1 to 5% by weight, the added amount of the amine in the paste is 0.5 to 10% by weight, and the added amount of the polyethylene glycol in the paste is 5 to 30% by weight. Particularly, in the water-based patch, diisopropanolamine is used as the amine.