A large scale MCFC (48 MW) was adopted to construct a high-efficiency and economic power generation system without $CO_2$ emission for utilizing its exhaust gas. A conventional steam turbine power generation system (STPS) is evaluated and the net generated power increase rate (NPIR) is estimated to be 8.22% and the STPS is economically feasible. A new $CO_2$-capturing repowering system with reheat cycle is proposed: in the proposed system, saturated steam produced at a HRSG by using the MCFC exhaust gas is utilized as a main working fluid of a gas ($H_2O$ gas) turbine, and the temperature of the steam is raised by combusting fuel in a combustor by using pure oxygen instead of air. It is estimated that the NPIR of the proposed system is 24.4%, and the $CO_2$ reduction amount is 27.0 kt-$CO_2$/y, compared to that (9.09 kt-$CO_2$/y) of the STPS, and that the proposed system surpasses the STPS in terms of economics.