To develop a new application for cross-laminated timber(CLT) in the civil engineering field, we focused on floor deck materials used for bridges and considered using CLT floor slabs to replace the floor slabs of existing bridges. When bridges are modified to meet modern design loads, in some cases, the primary girders require reinforcement owing to increased reinforcement concrete (RC) floor slab thickness. If the RC floor slabs can be replaced with CLT floor slabs of similar cross-sections, as CLT floor slabs have lesser weight, the modifications can be completed without reinforcing the girder. In this study, the feasibility of using CLT slabs is investigated for an existing forest roadway bridge in Hokkaido, where floor slab replacement work is planned, by analysis using the finite element method.