We present an adult case of the chest wall tumor, which was accidentally pointed out by a medical checkup. Surgical resection was performed for the tumor, as preoperative biopsy of the tumor suggested the possibility of malignancy. Postoperative pathological examination revealed the diagnosis of mesenchymal hamartoma of the chest wall, which usually occurs in early infancy and childhood. Immunohistochemical staining for Sox9 was positive for chondrocytes and partially positive for spindle tumor cells. It is considered that the present case was not pointed out until the patient became an adult, because the tumor was relatively small and thus asymptomatic.