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.

Inclusion Criteria:

• Patients who have been diagnosed as HAM according to the WHO criteria
• Patients who are positive for HTLV-I antibody in the spinal fluid
• Patients, if female, who are not pregnant or breastfeeding, either agreed to take contraceptive measures during and two years after the treatment, or sterile
• Patients, if male, who agreed to take contraceptive measures during and six months after the treatment
• Patients who have been informed and understood the contents of the study and consented to participate in the signed form.

Exclusion Criteria:

• Patients who has a rapid progress in the symptoms defined as an increase of two or more in Osame's Motor Disability Score for HAM patients in the past one year.
• Patients of hyperlipidemia (serum triglyceride higher than 400 mg/dL)
• Patients who were administered new or increased dose of corticosteroid in the past 8 weeks before the intervention
• Patients who received steroid pulse therapy in the past 8 weeks before the intervention
• Patients who were administered new or increased dose of immunosuppressant in the past 8 weeks before the intervention
• Patients with a history of serious drug allergy
• Patients with significant complication such as malignancy, severe heart failure, and other serious diseases.
• Patients who were in the past administered etretinate.