It is well known that the family-mold has an advantage to reduce the overall production cost. However, defects are frequently occurred by an excessive packing the smaller volume cavity during molding, especially when the volumetric difference between two cavities is not negligible. Although the size of runner could be optimized by CAE analysis, the filling imbalance is hardly avoided in the actual injection molding process by various means. In this study, we confirmed the filling imbalance in a family-mold by the temperature and the pressure sensors, and developed a variable-runner system for balancing the cavity-filling. We carried out experiments of balancing the cavity-filling in the family-mold with the variable-runner system, and examined the effect of reduced cross-sectional area of a runner in the system. We examined the conditions of the shear rate and the viscous heating in the system with a CAE analysis. We also examined the influence of the injection speed and types of resins to the balancing-capability of the system in order to help mold designers easily adopt the variable-runner system to their design.