Disclosed is an agent for preventing the reduction in the production of collagen, which comprises an enzymatically decomposed product of swallow's nest as an active ingredient, and is an agent for preventing the reduction in the production of collagen in a dermal fibroblast caused by the irradiation with an ultraviolet light. Specifically disclosed are: an excellent agent for preventing the reduction in the production of collagen in a dermal fibroblast, which can be produced with high efficiency and is highly safe； and an external preparation and a cosmetic, each of which comprises the agent.本發明之膠原蛋白產生低落抑制劑係一種真皮纖維母細胞因紫外線照射而導致膠原蛋白產生低落之抑制劑，且以燕窩之酵素分解物作為有效成分。依據本發明，可有效率地生產並提供安全性較高且抑制真皮纖維母細胞之膠原蛋白產生低落之效果優異的抑制劑，以及包含該抑制劑之外用劑及化粧品。
