It is conjectured that kyphosis in the elderly is affected by the ability to control dynamic posture. In this study, with quantitative assessment of kyphosis as the objective, we performed measurement of the index of kyphosis based on the method of Milne et al. which uses a flexible curve ruler, on elderly persons undergoing physical therapy at our hospital, and investigated its reliability and validity. The inter-and intra-class correlation coefficients were 0.858 and 0.851, respectively, both showing a high reliability, and according to the observations, a significantly high correlation was found in their relationship to kyphosis assessment, Spearman's rank correlation coefficient 0.819 (p<0.01). It is considered that the measurement of the index of kyphosis can be used clinically. Furthermore, according to the observations, since scattering of the index of kyphosis was found at each level of kyphosis assessment, it suggests that measurement of the index of kyphosis is of use as a method which can assess kyphosis in detail.