PROBLEM TO BE SOLVED: To provide a foamable external preparation for skin having improved transdermal absorbency of water-soluble active ingredient.SOLUTION: The foamable external preparation for skin comprises a water-soluble active ingredient in an external preparation for skin which generates carbon dioxide. In particular, it is preferable that the foamed external preparation for skin consists of two separated agents either one of the two agents comprises a bicarbonate, and the other comprises an acidic substance which generates foam by a reaction with the bicarbonate and either one or both of the two agents comprise at least the water-soluble active ingredient. Alternatively preferable is the foamed external preparation for skin consisting of one agent comprising at least a bicarbonate, an acidic substance generating carbon dioxide by a reaction with the bicarbonate, and the water-soluble active ingredient.
