The purpose of this study is to develop new standards to determine the sex of fragmentary human skeletal remains from archaeological sites in Japan. In order to accomplish this, we measured the long-bone circumferences of Japanese skeletons from the medieval period and provided metric diagnosis of sex using discriminant function analysis. We discuss whether the osteometric approach provides the criterion for sex assessment of human skeletal remains. The materials comprised human skeletal remains from the Yuigahama-minami site, Kamakura, Japan. The sample size used in this study was 68 males and 62 females excavated from individual burial graves. The accuracy of sex classification is more than 80% for discriminant functions with only one variable and reaches 90% for those with a combination of multiple variables. The multivariate functions provide better results than the univariate functions. Another improvement in sex diagnosis which this study contributes to is that new standards enabling reliable diagnosis with small numbers of variables are developed from well-preserved parts of the skeletons. This paper provides new standards, focusing on the diaphysial circumferences of limb bones from a medieval population, and will contribute to the advancement of medieval studies of skeletal remains from archaeological sites in Japan.