A 3-year-old boy with desmoid-type infantile fibromatosis arose in the mandible was reported. He was referred to our hospital because of suspected malignant bone tumor of the mandible. Histological examination of an open biopsy specimen was performed followed by tumor resection with marginal mandibulectomy and reconstruction by iliac bone grafting, which caused no functional complications nor mandibular deformity. To treat tumors in the facial skeleton, the surgical procedure should be planned based on the histological diagnosis in order to determine the proper area of resection to prevent functional or cosmetic complications. Especially in children, attention should be taken for benign but clinically resembling malignant rare diseases such as desmoid-type infantile fibromatosis.