The present invention provides: an agent for the treatment or prevention of HTLV-1-associated myelopathy (HAM), the agent containing an RGMa inhibitor; and a HAM treatment method including the administration of a pharmacologically effective amount of an RGMa inhibitor to an HTLV-1-associated myelopathy (HAM) patient in need of the agent.本發明提供一種HTLV-1關聯性脊髓病(HAM)之治療或預防劑,其包含RGMa抑制物質;及一種HAM之治療方法,其包括將藥理學上有效量之RGMa抑制物質向需要其之HTLV-1關聯性脊髓病(HAM)患者投予。