A novel, new, and highly effective agent for promoting fat degradation that contains astaxanthin and/or an ester thereof as active ingredient is provided. The agent for promoting fat degradation of the present invention has an effect of reducing fat by lipolysis of fat in either a subject who is in a condition in which fat has already accumulated or a subject who is in a condition in which fat has not yet been accumulated. Accordingly, the agent for promoting fat degradation of the present invention can restore hypertrophied adipocytes to a normal size.