The present invention relates to a composition for external application, a humectant and a skin barrier function reinforcing agent, each containing a diamide derivative represented by the following formula (1): ##STR00001## (wherein, R.sup.1a and R.sup.1b are the same or different and each represents a C.sub.1-23 hydrocarbon group, R.sup.2a and R.sup.2b are the same or different and each represents a divalent C.sub.1-6 hydrocarbon group, R.sup.3s are the same or different and each represents a divalent C.sub.2-6 hydrocarbon group and n stands for 1 to 100).The composition for external application, humectant and skin barrier function reinforcing agent basically improve the water retaining ability and barrier functions of the horny layer, are excellent in miscibility and mixing stability and can be prepared efficiently at a low cost.