In this study, the effect of exploiting an oscillatory motion of a manipulator on large force generation is demonstrated using simulations. Firstly, a natural frequency approximation method is devised, and a preferable posture of the manipulator is selected based on the approximation. The preferable posture is a posture in which the natural frequency of the manipulator is low. Since, in general, high frequency vibration is considered to be an undesirable phenomenon for mechanical systems, the manipulator should be operated in the preferable posture. Secondly, a method to oscillate the manipulator is proposed, and its performance is investigated using simulations. The method capitalizes on the oscillatory motion of the manipulator for efficient large force generation. Specifically, the method uses Van der Pol (VDP) oscillator to exploit an oscillatory motion of the manipulator. A force reference signal, which is a command to make the manipulator oscillate, is produced by the VDP oscillator. Due to the entrainment property of the VDP oscillator, the force reference signal can synchronize with the motion of the manipulator. The efficient large force generation is attained by the synchronization. Thirdly, a force control system that enables you to obtain the desired amount of force is designed based on the force generation method. By adjusting the natural frequency of the VDP oscillator, the purpose of the force control system is realized. Finally, a theoretical proof of the entrainment property of the VDP oscillator coupled with a linear mechanical system is established with an averaging method, and simulations exemplify the validity of the proof.