This paper describes the improvement of the maneuverability of a power assist device used in factories. An impedance control method based on force control is used in the device. Friction compensation can not applied in low velocity because the direction for the compensation is difficult to estimate in the velocity and the maneuverability becomes bad. Therefore, a technique in which the friction effect is reduced by mass effect is proposed for improving the maneuverability. If the mass of the power assist system becomes large, the friction effect is reduced but wasteful energy consumption becomes large. Then, an index evaluating both the reduction of friction effect and the energy consumption is applied to obtain an optimal mass and the effectiveness is shown by simulation.