In this study, the authors performed model tests and numerical analyses to clarify the quantitative effect of seismic countermeasures for existing mountain tunnels in poor geological conditions. It has become apparent that inner reinforcement can control the initiation of cracks and prevent spalling; backfill grouting can improve deformation performance of the tunnel and control compressive failure in arch crowns; and rock bolts can control heaving of the roadbed. Even though there are many restrictions associated with construction of inverts, it has become apparent that inverts are a fundamental countermeasure to control tunnel deformation and heaving of the roadbed.