A case of pigmented dermatofibrosarcoma protuberans (Bednár tumor) occurring in a Japanese infant is reported. A 6-month-old girl developed a 16 x 10 mm erythematous tumor with a slight elevation on the lower back at 1 month of age. Histological examination revealed proliferation of spindle-shaped, fibroblast-like cells in the myxoid interstitium. Subsequently the tumor grew gradually to a red-purplish tumor measuring 45 x 36 mm. The second biopsy specimen presented hypercellular proliferation composed of spindle-shaped cells showing slight nuclear atypia and a characteristic storiform pattern, with scattered slender dendritic cells containing a large amount of brown pigment. Immunohistochemical studies of the second biopsy specimen showed that the spindle-shaped tumor cells were positive for vimentin and CD34 and negative for factor XIIIa. The number of CD34-reactive spindle-shaped tumor cells increased in the second biopsy specimen compared with the first biopsy.