It is difficult to develop pharmaceutical agents for treating fibromyalgia since both the pathogenetic cause and the onset mechanism remain to be clarified. An antirheumatic agent salazosulfapyridine was administered to fibromyalgia patients. Unexpectedly, salazosulfapyridine was seen to be effective in fibromyalgia patients in whom neither the rheumatoid factor nor an immune disorder was detectable, in particular to relieve pain of enthesitis. Furthermore, salazosulfapyridine was also effective to alleviate enthesitis and to relieve pain in fibromyalgia. In addition, the combined use of salazosulfapyridine with a corticosteroid or a nonsteroidal anti-inflammatory agent potentiated the therapeutic effect of salazosulfapyridine in clinical patients to whom salazosulfapyridine alone was not effective enough.