Since 1950s, vibrations in the drive-rotor-effector system (rotor system) have been researched without losing interests. An application of rotor system, for example, a flywheel energy storage system, is recently focused on. Therefore the analysis of the rotor vibrations at the critical revolution speed still seems to be a substantial topic. In the mechanical engineering field, the dynamical behavior of the rotor is discussed by means of the simplified model, that is Jeffcott rotor. Most of the studies deal with the amplitude of the whirling motion of the rotor under the assumption that the revolution speed of the rotor is regarded as a input. In general, however, the input from a drive (i. g., motor, engin, and so on.) to the rotor is torque. So it seems important to discuss the behavior of the system under the situation that torque is engaged as a input to the rotor. In this paper, a whirling motion in a DC motor-rotor-DC generator system used as a rotor system is experimentally discussed based on the output voltage of the generator. The results show that at the mechanical resonant state, the hunting appears to the revolution speed of the rotor even in the constant input torque situation. Furthermore it is clarified that the output voltage shows quasi-periodicity, multifractal, mode-lock and chaotic oscillations, and the existence of hysteretic jumps at the appearance and the disappearance of the oscillation.