This is a case report on stereotaxic (Stereotactic Body Radiotherapy-SBRT) for lung cancer located in the left lower lobe (Segment 6, S6). There have been no reports on marked displacement of the peripheral lung cancer during radiotherapy. A pulmonary nodule was discovered on computed tomography (CT) conducted for a persistent cough in an 87-year-old male. According to diagnostic imaging, this nodule was clearly delineated and had an irregular margin. The image diagnosis was T1N0M0, stage I primary lung cancer located in the left lower lobe and no pathological type was identified. The patient was treated with SBRT using a Linear accelerator (LINAC) at a total dose of 48 Gray (Gy) in 4 fractions. On performing cone beam CT (CBCT) at the third fraction, the tumor position had moved caudally by about 3 centimeters (cm). For this reason, we canceled further treatment and the case was re-planned. Four days after discontinuing treatment, 4-dimensional CT (4DCT) images were obtained before initiation of the remaining fractions of SBRT. Therefore, the patient completed 4 fractions of SBRT and the tumor location was confirmed before beam delivery. The tumor location differed based on the patient’s position (i.e., standing versus lying in a supine position), and we considered that it moved due to adhesion to the pleura when the patient was standing. If reproducibility of the tumor position cannot be guaranteed, the patient should undergo to CT re-simulation. Oncologists should re-evaluate the movement of the tumor on respiration and adjust the margins accordingly.