The present invention addresses the problem of providing a means which is effective for the induction of the differentiation of a brown adipocyte and therefore enables a brown adipocyte to be used for and applied to, for example, the prevention/treatment of obesity or metabolic syndrome. Provided is a brown adipocyte differentiation-inducing agent containing, as an active ingredient, (1) CREG1 protein or (2) an expression vector carrying CREG1 gene.