This study proposes a new mathematical model for the amplification ratio of a single-stair bridge-type amplification mechanism. To overcome the limitations of previous models, all members of the amplification mechanism are assumed to be compliant. The proposed model is compared to conventional models as well as analysis results using finite element models, after which the effects of the dimensions of each member are investigated. In addition, design optimization is carried out. Finally, the usefulness of this mathematical model is verified by an experiment using the optimally designed mechanism. As a result, the proposed mathematical model very aptly describes the displacement amplification ratio of the single-stair bridge-type amplification mechanism.