[PROBLEM] The problem to be solved by the invention is to provide a novel pharmaceutical use of a JAK inhibitor. [SOLUTION MEANS] A therapeutic or preventive agent for a skin disease selected from the group consisting of senile xerosis, asteatosis, eczema and contact dermatitis, containing a JAK inhibitor as an active ingredient. [EFFECT] The followings are found : a JAK inhibitor increases the expression amounts of filaggrin, loricrin, involucrin and ?defensin 3 as skin barrier function-related proteins; a JAK inhibitor significantly increases NMF production in a Tape Stripping-treated mouse; and a JAK inhibitor significantly accelerates a reduction in TEWL in a dry skin mouse model, namely improves the skin barrier function. The JAK inhibitor can be used as an active ingredient of a therapeutic or preventive agent for skin diseases such as senile xerosis, asteatosis, eczema, contact dermatitis, ichthyosis vulgaris, Netherton syndrome, type B peeling skin syndrome, etc.