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Traction drive is a useful power transmission method for reducing noise of high-rotational-speed motors of electric vehicles. To realize a highly efficient traction drive system, normal force between rollers must be optimized. This study uses a piezoelectric actuator to adjust the normal force. A piezoelectric actuator, which deforms according to the applied voltage, can output large force. Nevertheless, piezoelectric actuator cannot be integrated with a force sensor that has low stiffness. Therefore, we have developed a normal force control method that observes the slip between rollers rather than the normal force. A two-roller test bench has been developed to confirm its efficiency. Experimentally obtained results demonstrate that the proposed system can achieve considerably high efficiency.