Not only is the concurrence of congenital myotonic dystrophy (CDM) and subgaleal hematoma (SGH) hardly ever seen but also the development of SGH during unassisted vaginal delivery is rare. We report a boy who developed huge SGH in vaginal delivery without any use of vacuum or forceps and later was diagnosed as maternally transmitted CDM. The boy had prenatal history of polyhydramnios and decreased fetal movement. Six hours after birth, severe molding of the skull associated with huge SGH on left parieto-occipital area was recognized by CT scan. At corrected age of two months, he was diagnosed as maternally transmitted CDM. This is the first report of CDM complicated by SGH occurring in non-instrumental vaginal delivery.