Provided is a water-based plaster in which the gel strength is maintained to an appropriate level in a step of producing the water-based plaster, and particularly in a spreading step and an aging step, and a final preparation can exhibit an optimal adhesiveness for adhering to the skin. In the water-based plaster, a water-based adhesive base containing a water-soluble polymer as a main ingredient contains a higher fatty acid which is a liquid at room temperature and a viscosity of 1,000 (mPa·s, 20°C) or more as a crosslinking modifier. The higher fatty acid is isostearic acid, and the contained amount thereof is 1 to 10% by weight.
