Provided is a water-based plaster whereby the gel strength of a water-based adhesive base can be retained in an appropriate state during the steps for producing the water-based plaster, in particular a spreading step and conditioning step, and whereby an optimal adhesiveness for adhering to the skin can be exhibited in a final preparation. The present invention is a water-based plaster, characterized in that a water-based adhesive base in which the main ingredient is a water-soluble polymer contains, as a crosslinking modifier, a higher fatty acid which is liquid at room temperature and has a viscosity of 1000 (mPa·s, 20°C) or higher, wherein the higher fatty acid is isostearic acid and the formulated amount thereof is 1-10 wt%.
