A 18-year-old man was referred to our hospital complaining of chest abnormal shadow of the left upper lung field in mass screening chest X-ray. Although his left anterior chest wall swelled, he did not recognize that. Computed tomography and magnetic resonance imaging demonstrated a dumbbell shaped tumor of the left 1st intercostal space which had grown both inside and outside the thoracic cavity. As a possibility of solitary fibrous tumor or myxoma was not excluded, the patient underwent tumor resection. A solid tumor connected to the 1st intercostal nerve was easily dissected from surrounding tissue. Pathological examination revealed the tumor was consisted of spindle shaped cells without atypia, and diagnosed as neurilemmoma without malignancy. Based on anatomical pathway of intercostal nerves, we speculate that the tumor originated from anterior cutaneous nerve.