A 34-year-old man had a 3-month history of left upper eyelid swelling and pain. Computed tomography showed osteolytic lesion of the left frontal bone. Magnetic resonance imaging (MRI) revealed a mass lesion with heterogeneously gadolinium enhancement and extension to both the lateral orbit and the frontal base. We performed mass resection via left frontotemporal craniotomy to diagnose the lesion. Pathological findings of the specimen were Langerhans cell histiocytosis (LCH). Systemic investigations revealed no sign of the disease elsewhere. The patient was diagnosed with unifocal orbital LCH and received neither chemotherapy nor radiotherapy. Follow-up MRI has shown no recurrences 2 years after surgery. Although orbital LCH rarely occurs in adults, it should be considered in differential diagnosis of orbital tumor. The patients with unifocal orbital LCH required no treatment in addition to mass resection.