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The Ba<sub>2</sub>Ca<sub>7</sub>Cu<sub>8</sub>O<sub>16</sub>(O<sub>0.8+δ</sub>F<sub>1.2</sub>) (F-0278) superconductor with 8 CuO<sub>2</sub> 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>i.e.</i>, 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.