This paper introduces an exoskeleton for reduction in a metabolic cost during running. The exoskeleton aims to reduce load of runner's vertical oscillation of center of mass. The exoskeleton consists of a leaf spring, a slider and a brake unit for holding and releasing the slider of each leg. The exoskeleton holds the slider during a stance phase and releases the slider during a swing phase. For assisting the oscillation, the leaf spring stores energy during an initial stance phase and restore the energy during a terminal stance phase. A heart rate of a subject shows assistance effect of the exoskeleton through running.