Behavior and shapes of a pair of bubbles rising in a viscous liquid were numerically analyzed by using DSD/ST (Deforming-Spatial-Domain/Space-Time) Finite Element Method in the cylindrical coordinate system. It has been predicted by the analytical and numerical studies that the trailing bubble approaches the leading bubble and then a pair of bubbles rising in the vertical line keeps the mutual equilibrium distance. However, no experimental results have ever confirmed the existence of the equilibrium distance between a pair of bubbles. In this study, we investigated the effects of bubble deformation on behavior of a pair of bubbles rising in line. We clarified that a pair of deformable bubbles reaches an equilibrium distance and the equilibrium distance is considerably unstable.