A kappa-opioid receptor agonist comprising a 2-phenylbenzothiazoline derivative which is either a compound having a basic skeleton having a chemical structure represented by the general formula [I] or a salt of the compound. The presence of an amino-substituted alkyl group bonded to the phenyl group of 2-phenylbenzothiazoline and the presence of an acyl group bonded to the nitrogen atom of the 2-phenylbenzothiazoline are important for the impartation of kappa-opioid receptor agonistic activity. (I) (In the formula, R represents amino-substituted alkyl and R<1> represents acyl.)
