The 2017 revision of the Specifications for Highway Bridges in Japan mandates that the load-bearing capacity of floor slabs in consideration of Level 2 earthquakes must be verified. By this, the Japan Bridge Construction Association’s “Design Examples and Explanations for Composite Girders” report illustrates that FEM analysis reveals the load-sharing ratio of the slab to be 80%. However, the “Design Examples and Explanations for Narrow Box Girder Bridges” report demonstrates that FEM analysis results indicate the slab’s ability to resist 100% of the loads, ensuring the safety of the design. It is essential to note that the “Design Example and Commentary” assumes that all lateral loads acting on the superstructure are borne by the slab, enabling the bridge to be safely designed. However, when the lateral load verification based on these design examples is verified at the practical design, there are cases where the results deviate from the conventional practical design results. Therefore, it is imperative to assess the load-sharing ratio based on the actual behavior of the bridge to achieve a rational design. This study aims to evaluate the load-sharing ratio of the slab and main girder by FEM analysis, based on a design example. The analysis method takes into account material nonlinearity and illustrates the load-sharing ratio at each design horizontal seismic intensity. The analysis results indicate that the load-sharing ratio of the main girder increases after the design horizontal seismic intensity of 0.8, while the load-sharing ratio of the slab tends to decrease. In the bridge case of this study, a load-sharing ratio of 0.75 for the slab and 0.20 for the main girder is appropriate. However, the points to keep in mind are that if the nonlinear properties of the concrete are distributed in the plastic range, the load-sharing ratio will be different.