The primary objectives of the study are:

• To evaluate the safety and tolerability of MT-1303 in subjects with moderate to severe active Crohn's Disease（CD）
• To evaluate the clinical efficacy of MT-1303 in subjects with moderate to severe active CD.

Inclusion Criteria:

• Diagnosis of CD (involving small intestine and/or colon), confirmed by both endoscopy and histopathology at least 3 months prior to Visit 1
• Previous use of any type of corticosteroids or immunosuppressants for the treatment of CD
• Moderate to severe active CD defined by a CDAI score of ≥220 to ≤450 points at Visit 1

Exclusion Criteria:

• Diagnosis of ulcerative colitis, indeterminate colitis, pseudomembranous colitis or coeliac disease
• Enterocutaneous, abdominal or pelvic active fistulae, abscesses or fistulae likely to require surgery during the study
• GI surgery (including appendectomy) within 12 weeks prior to Visit 2 (Baseline) or has surgery planned or deemed likely to require surgery for CD during the study