PROBLEM TO BE SOLVED: To provide a highly safe and efficiently producible agent for excellently preventing the reduction in the production of collagen in a dermal fibroblast, and to provide an external composition for the skin and a cosmetic each containing the agent.SOLUTION: The agent for preventing the reduction in the production of collagen includes an enzymatically decomposed product of a swallows nest as an active ingredient. The agent prevents the reduction of the production of collagen caused by ultraviolet irradiation in the dermal fibroblast.