The purpose of this study was to build a flexible mechanical system with a hydrostatic skeleton. The main components of this system are two type flexible bags. One is a structural bag with constant inner pressure. The other is an actuator bag with controlled inner pressure. To design the system, it was necessary to estimate both structural deformation and driving force. Numerical analysis of flexible bags, however, is difficult because of large nonlinear deformation. This study analyzed structural strength and driving force of flexible bags with the nonlinear finite element analysis (FEA) software ABAQUS. The stress concentration dependency on the bag shape is described and the driving force is calculated to include the large deformation. From the analytical results, this study derives an empirical equation of driving force. The validity of the equation was confirmed by condition-changed analyses and experimental results.