A method of screening a substance which serves as the active ingredient in a drug for obesity and/or diabetes is provided. This method comprises treating cells or an animal individual with a candidate substance and specifying a candidate substance fulfilling one or more of the following requirements as the target active ingredient substance: (a) increasing the expression level of a ligand factor ERRL1 for a nuclear receptor ERR; (b) increasing the transcriptional activity of the nuclear receptor ERR; (c) promoting the binding of ERRL1 to ERR; and (d) increasing the expression level of an MCAD gene product.