[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 β-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.
