The present invention provides a pharmaceutical composition which can be used for the treatment of HTLV-1-associated myelopathy and contains an inhibitor selected from the group consisting of an EZH1 inhibitor, an EZH2 inhibitor and an EZH1/2 double inhibitor as an active ingredient. The present invention provides a pharmaceutical composition which can be used for the treatment of HTLV-1-associated myelopathy and contains a 1,3-benzodioxole derivative or a pharmaceutically acceptable salt thereof.本發明提供一種用以用於治療HTLV-1(Human T-lymphotropic Virus Type 1)相關脊髓病之醫藥組成物,其包含選自包含EZH1抑制劑、EZH2抑制劑及EZH1/2雙重抑制劑之群組的抑制劑作為有效成分。本發明提供一種用以用於治療HTLV-1相關脊髓病之醫藥組成物,其包含1,3-苯并二呃(1,3-benzodioxole)衍生物或其醫藥上可容許的鹽。