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In controlling a hopping robot, reducing reaction force from the ground is important. We propose a method for a hopping robot to reduce impact force at the instant of touchdown. Our method is based on the idea of the soft-landing trajectory and optimal approach velocity. By applying our method to a robot system, a robot can suppress the impact force at the instant of touchdown and press the ground with constant force continuously. Furthermore, selection ratios of position and force are introduced to achieve a smooth transition from force control to position control during the landing phase. It is important for a hopping robot to achieve a desired position when the position of the COG (center of gravity) is the lowest; because the robot have to prepare for a next hop. With this method, the impact force at touchdown is suppressed and the robot can track the desired trajectory. The method is applied to a robot which is constrained to move only vertical direction. Validity of our method is confirmed by numerical simulations.