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.
