The present invention provides κ opioid receptor agonists comprising 2-phenylbenzothiazoline derivatives. The present invention relates to κ opioid receptor agonists comprising compounds or salts thereof having the chemical structure represented by the general formula [I]. Namely, it is important for exhibition of the κ opioid receptor agonist actions to have an alkyl group having an amino group at a phenyl group of 2-phenylbenzothiazoline as a substituent and to have an acyl group at a nitrogen atom of 2-phenylbenzothiazoline, wherein R is alkyl having the amino group as the substituent and R1 is acyl.
