The study will verify the superiority of MT-0551 to placebo at 26 weeks after treatment initiation in systemic sclerosis (SSc) patients using the modified Rodnan Total Skin thickness Score (mRTSS) as a measure of skin thickening. The safety and pharmacokinetics will also be investigated.

Inclusion Criteria:

Additional screening criteria check may apply for qualification:

1. ACR/European League Against Rheumatism (EULAR) criteria (2013) systemic sclerosis (SSc) diagnostic criteria satisfied.
2. Skin thickening score based on the mRTSS between 10 and 22 inclusive.

Exclusion Criteria:

Additional screening criteria check may apply for qualification:

1. Pulmonary hypertension associated with SSc.
2. Presence of a serious, SSc-related concurrent illness other than interstitial pneumonia.
3. Finding of inadequate respiratory reserve capacity.
4. Past history of rituximab, blinatumomab, obinutuzumab, or ofatumumab.
5. Presence of a clinically significant active infection requiring antimicrobial therapy.
6. A past history of cancer.
7. Past history of a recurrent, clinically significant infection.
8. Past history of severe allergy or anaphylactic reaction to a biologic drug product.
9. Treatment with live vaccine within a certain period (inactivated vaccine is acceptable).