In Japan, the three major treatment options for gastric cancer are gastrectomy, endoscopic mucosal resection, and anticancer drugs (or combinations of the three). The surgical procedures are well established and the results are satisfactory, but there is no prospect of extending surgical approaches beyond the techniques that already exist. On the other hand, remarkable developments have been made in chemotherapy and molecularly-targeted therapy, and these have contributed to a reduction in deaths from gastric cancer. However, patients with gastric cancer that progresses to a certain level are classified as having systemic disease with an increased risk of postoperative recurrence. Therefore, adjuvant chemotherapy aimed at preventing recurrence, such as metastases or peritoneal dissemination, after surgery can be said to be an indispensable partner of surgical treatment. S-1 is still a key drug in gastric cancer treatment, and maintenance of nutrition and skeletal muscle mass after gastrectomy are important for the continuous use of S-1 after surgery. Taking skeletal muscle mass measurements after surgery is a useful and simple was to predict prognosis.