The present invention discloses novel piperidine and pyrrolidine derivatives having NPY Y5 receptor antagonistic activity. Specifically, the present invention discloses a compound represented by the formula (I), a pharmaceutically acceptable salt or a solvate thereof:whereinA is substituted or unsubstituted monocyclic aryl or monocyclic heterocyclyl； X is a single bond or C(=O)； Y is a single bond, CR5R6NR7 or C (=O) NR7； R1 and R2 are independently hydrogen or substituted or unsubstituted alkyl； R3 is substituted or unsubstituted aryl or heterocyclyl； R4 is halogen, substituted or unsubstituted alkyl, alkoxy, aryloxy or heterocyclyloxy, etc.； p is an integer of 0 to 2； q is 0 or 1； m is 0 or 1； n is an integer of 0 to 5； and B is aromatic carbocycle, monocyclic heterocycle or bicyclic fused hetero ring.
