When an elevator rope for a high-rise building is forcibly excited by long-period ground motion, rope displacement becomes large even if the ground acceleration is small. In this paper, a simplified calculation method, based on a single degree of freedom (SDOF) system, for estimating rope displacement of moving cage during an earthquake by using the building's acceleration and ground acceleration is presented. Finite difference analyses of rope vibration are also performed to verify the validity of the simplified method. The results of this simplified calculation method agree with those of the conventional finite difference method, within the 20% error margin.