A strain wave gearing (commonly known as harmonic drive gearing) has been widely used as the reduction gear drive in industrial robots. Assembling an industrial robot system with an input motor, a strain wave gearing and an output arm, the amplitude of vibration in the robot system had been measured previously. Translating the robot system into an equivalent vibration model, the vibration occurred in the robot system was analyzed theoretically to compare the theoretical value with the experimental one and to determine the cause of the vibration with following result : The amplitude of the vibration occurred in the robot system was influenced by the variation of the spring stiffness and by the composite error in meshing, due to the machining and assembly errors of elements.