For bifacial solar cells, a gain of 10-50% of the electric power generation by harvesting albedo radiation is expected compared with mono-facial solar cells. Cu(In,Ga)Se-2 (CIGSe) is a promising photovoltaic material owing to its unique physical properties, whereas the knowledge of the bifacial CIGSe solar cells is not fully understood. In this article, high efficiencies under front and rear illuminations for the bifacial CIGSe solar cells are theoretically demonstrated. An adequate bandgap grading by Ga/(Ga + In) ratio in the CIGSe layer leads to a shift of the depth in carrier-generation toward pn-junction and driving force of electron transport by electric field, resulting in improvement of device performance. On the other hand, it is also disclosed that the effect of bandgap grading alone is not enough to obtain the high short-circuit current density (J(SC)) under rear illumination. It is found that improvement of a CIGSe quality and an introduction of a rear surface layer contribute to reduction of the carrier recombination at the bulk and back contact, respectively, leading to enhanced J(SC) in rear illumination. Furthermore, the effect of optical path length by the surface texture is discussed. These findings indicate a guideline for the promising high-performance bifacial CIGSe solar cells.