Floating gear mechanism is widely used in planetary gear trains to share a transmitted load equally with planet gears. Our experiments show that this mechanism causes an abnormal lateral vibration of the floating gear when a large torsional resonance occurs. This paper proposes a dynamic model and simulation method for analyzing the abnormal lateral vibration of floating sun gear shaft in a star-type planetary gear train. The dynamic model is composed of a motor, planetary gear train and dynamometer, which is the same as our experimental apparatus. The total degree of freedom of the model is 15. The gears are connected each other by 12 springs with backlash modeling the mesh between gear teeth. An pulsating torque of the motor is considered. The simulated results are in good accordance with the experimental ones as for vibration wave forms and resonance curves and also show how to prevent the abnormal vibration. © 2000, The Japan Society of Mechanical Engineers. All rights reserved.