There are provided novel 4-(2-furoyl)aminopiperidines represented by the general formula (I), their synthetic intermediates, processes for their preparation and medicaments containing them. In the above formula, X is CH or N, and Y is a group of the following general formula (II), formula (II-a) or formula (III): wherein a, b and c are each an integer of 0-6; Z is CH2 or NH; W is O or S; T is O or N—R15 wherein R15 is H, a C1-C6 alkyl group, a benzyl group or a phenethyl group; and R1 is H, a C1-C6 alkoxycarbonyl group, a benzyloxycarbonyl group, or the like. The 4-(2-furoyl)aminopiperidine derivatives according to this invention possess opioid μ antagonistic activity and are useful for the treatment or prevention of side effects which are caused by μ receptors agonist and which are selected from constipation, nausea/emesis or itch, or for the treatment or prevention of idiopathic constipation, postoperative ileus, paralytic ileus, irritable bowel syndrome or chronic pruritus.