An object of the present invention is to provide: a novel base that, after application to a skin surface, can rapidly form a coating film and moreover mitigates skin irritation and occurrence of odors and an external preparation for skin. The base is used in an external preparation for skin that administers an active ingredient transdermally, and is characterized by containing at least an acrylic-based synthetic polymer and a volatile oil, wherein the base is in a liquid or ointment form prior to application to the skin surface, and the volatile oil evaporates after application of the base to form a hydrophobic coating film containing the volatile oil in the range of 0 to 60 wt % with respect to 100 parts by weight of the acrylic-based synthetic polymer.
