Myeloid sarcoma, which sometimes accompanies recurrent systemic leukemia, is a rare manifestation of congenital leukemia. We describe the case of a 1-month-old Japanese girl with myeloid sarcoma, who did not fulfill the criteria of systemic leukemia. The physical examination revealed a bruise-like plaque on the right side of her chest. Histopathologically, there were CD4+ CD56- CD68+ atypical large lymphocytes densely infiltrating the upper dermis and the subcutaneous tissue. Biopsy specimens from bone marrow contained only 3.4% of blast cells, showing an abnormality of the mixed-lineage leukemia gene. The skin lesions disappeared 10 days after chemotherapy, and there was no recurrence for one and a half years.