(R)-2-(2-aminothiazol-4-yl)-4′-[2-[(2-hydroxy-2-phenylethyl)amino]ethyl]acetic acid anilide or its salt shows a potent bladder relaxation effect in “isolated rat bladder smooth muscle relaxation test”, dose-dependently lowers the contraction frequency of rhythmic bladder contractions in “rat rhythmic bladder contraction measurement test” and, moreover, prolongs the urination intervals in “urination functions measurement test on cyclophosphamide-induced overactive bladder model rat”. Owing to these effects, the above compound is useful as a remedy for overactive bladder.