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This study will address the improvement of the analytical accuracy of a lateral vibration analysis model for rotor-dynamics. In case of predicting the lateral vibration of a rotor with disc elements fixed on the shaft by the interference fit, the accuracy of the bending stiffness of the corresponding shaft element could affect the accuracy of the predicted vibration property. This paper describes the investigation results for estimating the bending stiffness of a rotor with a sleeve fixed on it by the interference fit using the finite element analysis model which can treat the contact problem including the friction effect. The results show that the bending stiffness of a shaft element with a sleeve depends on the occurence of the relative slip on the fitting surface, and the bending stiffness is equivalent to the one of a solid shaft element that has the same outer diameter as the sleeve when the relative slip does not occur. Additionally, we have evaluated the natural frequencies of a shaft with a sleeve on it using the improved 1D finite element model and the vibration test. The approximate critical amount of the bending moment which will cause the relative slip on the fitting surface is estimated by the analysis. Comparing this amount of to the bending moment caused by gravity and unbalance force acting on the horizontal rotor, we have obtained the permissible quality factor Q in resonance for a given balance quality.