The Ba2Ca7Cu8O16(O0.8+δ F1.2) (F-0278) superconductor with 8 CuO2 planes in the structural unit was investigated by analyzing the reversible magnetization based on the Hao-Clem model and fluctuation theories. In spite of the superconducting block (SCB) being thicker than the charge reservoir block (CRB), the analysis clearly revealed two-dimensional (2D) superconducting properties in F-0278, such as a strong temperature dependence of the Ginzburg parameter κ and a relatively short effective interlayer distance. The extreme 2D behavior can be understood in terms of the significant charge imbalance in the SCB, which was previously proposed to exist in F-0278, where the carriers are distributed quite unequally to crystallographically different CuO2 planes; i.e., the outer planes (OPs) adjacent to the CRB always contain more carriers than the inner planes (IPs). The 2D nature of F-0278 indicates a significant or complete suppression of the superconductivity in the IPs. The important superconducting parameters were also obtained from the analysis.