An interface is formed by incorporating (1) a mixture comprising a water-soluble protein and a drug into a matrix, or preferably (2) a mixture comprising a water-soluble protein and a drug to a porous matrix coated with a cationic surfactant or other ionic surfactant. The coating amount of the ionic surfactant is about 0.1 to 50 .mu.g, and the content of the water-soluble protein is about 0.1 to 1,500 .mu.g, each per 1 cm.sup.2 of the matrix. The water-soluble protein includes an albumin and the drug includes a physiologically active peptide or protein, for example. The use of the stabilized interface inhibits decrease of the drug retaining amount, and insures an effective transdermal drug delivery with a high repeatability and accuracy.