The temper rolling process of hot-rolled strips is the final rolling step to improve the flatness of strips and shape slippage of coiled strips. Non-flatness of hot-rolled strips causes the strip lateral movement during the temper rolling process. Manual leveling operations, the quality of which depends on the operator’s experience, control this movement but result in much lower line speed and productivity. A lack of understanding of this phenomenon has sustained manual operation and discouraged the development of an automatic control system. In this paper, both a new lateral movement model, where the temper-rolling lateral movement model and the large-deflection strip model are important parts, and a theoretical method, where lateral movement stability is equivalent to the eigenvalue problem with lateral movement static equations, are proposed. Their usefulness is confirmed by comparing the results of experimental rolling in the laboratory with those of numerical calculations. Simulation results obtained using the proposed models confirm that actual problems are solved more exactly than with the conventional linear model. The simulation can support the clarification of lateral movement and the development of an automatic control system.