A P2X4 receptor antagonist such as paroxetine, a diazepinedione derivative having the following formula (IX) is used as an agent for preventing or treating neuropathic pain associated with Guillain-Barr?syndrome : wherein R1 is hydrogen, a C1-8 alkyl group, or the like; each of R2 and R3 is hydrogen, a C1-8 alkyl group, or the like; each of R4 and R5 is hydrogen or the like; andW is a five-membered or six-membered heterocyclic ring optionally having one or more substituents and comprising one to four nitrogen atoms as the members of the ring.