To provide an external skin agent that prevents onset of atopic skin inflammations and is low irritant to skin, which is characterized by including:1) clay modified by a compound having a quaternary amino group;and 2) a polymer or copolymer that has a side chain including a structure similar to a biological component.提供一種預防異位性皮膚炎症發作之外用皮膚劑且對皮膚為低刺激性,其特徵為包括:1)以具有四級胺基之化合物修飾之黏土;及2)具有側鏈包括相似於生物組份之結構之聚合物或共聚合物。