Congenital melanocytic nevi are considered to be precursors of malignant melanoma. Although the risk of malignant melanoma with medium and small congenital melanocytic nevi is uncertain, it is important to notice the possibility of malignant transformation in those lesions. We describe a 62-year-old woman who had had a brown soft verrucous tumor on her right lower back since birth. She first noticed a black nodule in the center of the tumor 5 years before which had ulcerated 3 months prior to presentation without healing. A biopsy specimen revealed malignant melanoma arising from a congenital melanocytic nevus.