Power assist devices are attracting remarkable attention as they can reduce the burden of workers engaged in heavy labor: nursing care, manufacturing, logistics, and farming. However, the effect of power assist devices in reducing labor’s fatigue has not been sufficiently evaluated. The purpose of this study is to quantitatively evaluate the assistive effect of power assist suit in terms of energy consumption. In the experiment, we particularly focused on the effect of Muscle suit® which was developed to reduce the burden on back in lifting heavy weight. The measurement results of energy consumption show that Muscle suit can reduce labor’s fatigue from the perspective of energy metabolism.