We have developed a hip exoskeleton for seniors with difficulties in walking due to muscle weakness. The exoskeleton is lightweight and moderate in assistance power compared to other hip exoskeletons in the literature. Its controller estimates user gait phase, walking speed, and ground inclinations to generate assistance torque adaptively. To assess the physiological effect of the gait assistance, we compared metabolic energy consumption for 5 adults for walking on a treadmill with and without the exoskeleton at the same speed: the exoskeleton reduced metabolic cost of walking by 13% (p = 0:0024). The step length and the stride time increased under the assistance. Our analysis for the result suggests that the efficiency of hip exoskeletons on saving metabolic energy can be twice as high as that of ankle exoskeletons possibly because muscle-tendon unit in the hip joint is less energy-efficient than in the ankle joint.