The patient was a 66-year-old woman. She underwent central venous port insertion as part of postoperative adjuvant chemotherapy for sigmoid colon cancer. At the beginning of the 2 cycle, she experienced discomfort in the neck, and computed tomography was performed. As a result, catheter deviation and a thrombus in the internal jugular vein were observed. It was considered that breast displacement due to gravity caused the catheter deviation and that the position of the tip of the catheter deviating to immediately above the venous valve caused thrombus formation. We examined the factors that may cause catheter deviation.