Skin external agent compositions and drugs characterized by containing fumaric acid diester derivatives represented by following general formula (1), wherein R¹ represents C_1-4 alkylene; R² and R³ represent each liner or branched C_1-8 alkyl; and n is an integer of 2 to 5; which provide epidermal barrier function potentiating agents and skin external agent compositions having a high safety, quickly relieving epidermal permeation barrier function failures and having an effect of maintaining the skin in a cosmetologically healthy state; agents to be used together with HB-EFG expression inducing agents accelerating epidermal turnover, amphiregulin expression inhibitors regulating excessive epidermal proliferation, and retinoic acid or its derivative preventing skin chapping caused by retinoic acid or its derivative; and skin aging relieving agents capable of ameliorating depression in the skin metabolism and thinning in association with aging.