An open-label, non-randomised, uncontrolled, proof-of-concept study of patients with HTLV-I-associated myelopathy/Tropical Spastic Paraparesis (HAM/TSP). Participants will receive oral administration of tamibarotene in the amount of 2 mg daily over a period of 12 weeks, then 4mg daily for another 12 weeks. The patients will be followed up for further 8 weeks. Efficacy will be monitored by measuring clinical scores including motor and urination function, HTLV-1 proviral load, immunological parameters, and markers in the spinal fluid. Safety will be evaluated at the same time.