We present a 30-years-old male case of insufficiency fracture of the bilateral femoral neck. He suffered from bilateral femoral neck fracture without a definite trauma.Radiological examination on the first visit showed a minute changes in both femoral necks, which were inferred fracture retrospectively. On the second visit, radiogram revealed obvious desplaced femoral neck fractures. The patient was therefore admitted into our hospital and fractures were fixed with cannulated screws. At admission, laboratory date revealed no definite abnormalities, except bone atrophy in bone mineral analysis. These findings suggested that idiopathic osteoporosis acts as one of the factors which caused the insufficiency fracture.