The purpose of this study is to develop a new system for measuring leg motions using a portable three-dimensional accelerometer. The measuring system is composed of acceleration sensors (Micro Stone, MA3-10Ac), a data logger, a data reader, and a personal computer. The personal computer draws a graph of the acceleration of movements (i.e., accelerogram) from the output signals of the acceleration sensors. We then calculated the average acceleration to evaluate leg motions. We drew the accelerograms from 19 young subjects and 36 elderly subjects during the performance of physical fitness activities and walking. Different accelerograms were obtained from two subjects for different walking styles. The average accelerations at the lumbar, ankle and toe points were higher at all axes during movements from the physical fitness activities in the elderly subjects. The accelerations of leg motion at the knee point were, however, lower in the up and down axis in the elderly subjects than in the young subjects, although they were higher in the back and forth and the right and left axes. The new instrument has enabled us to evaluate leg motions by measuring three-dimensional acceleration during the performance of physical fitness activities and walking. The average accelerations of leg motions showed age-related changes. Thus, the average acceleration of leg motions may be used as a new index for evaluating leg motions at the dynamic state.