Piperidine compounds prostaglandin D synthase inhibitors

A piperidine compound represented by Formula (I) ** ** Formula wherein X1, X2, and X3 are the same or different and each represents N or C-R1; n is 0 or 1; A is phenylene, a saturated heterocyclic group, divalent heterocyclic group or divalent unsaturated; B is hydrogen, halogen, alkyl which may have a substituent, alkenyl which may have a substituent, phenyl which may have a substituent, aralkyl which may have a substituent, heteroaralkyl may have a substituent, a saturated heterocyclic group which may have a substituent , a heterocyclic group which may have a unsaturated substituent, NR2R3, (C> = O) R4, or O-R5; R1 is hydrogen, halogen, or alkyl; R2 and R3 are the same or different and each represents hydrogen, phenyl, alkylcarbonyl, alkoxycarbonyl saturated or unsaturated heterocyclic, phenylaminocarbonyl, or alkoxycarbonyl; R4 is substituted alkyl, cycloalkyl, trifluoromethyl, phenyl, unsaturated heterocyclic group, heteroaralkyl, saturated heterocyclic group, or NR6R7; R5 is phenyl, aralkyl, or an unsaturated heterocyclic group; R6 and R7 are the same or different and each represents hydrogen, alkyl, cyclohexyl, phenyl which may have a substituent, an unsaturated heterocyclic group, aralkyl, or heteroaralkyl; or R6 and R7, taken together with a nitrogen atom to which they are attached, form a pyrrolidyl group or a piperidyl group; or a salt thereof.