A combined power generation system includes a gas turbine, a heat recovery steam generator (HRSG) configured to heat feedwater using combustion gases discharged from the gas turbine and having a high-pressure section, a medium-pressure section, and a low-pressure section having different pressure levels, an ammonia decomposer decomposing ammonia with the combustion gases discharged from the gas turbine, a first exhaust gas line through which the exhaust gases discharged from the gas turbine are transferred to the HRSG, a second exhaust gas line through which the exhaust gases discharged from the gas turbine are transferred to the ammonia decomposer, a third exhaust gas line through which the exhaust gases discharged from the ammonia decomposer are transferred to the HRSG, and a decomposed gas transfer tube connecting the ammonia decomposer and the gas turbine of a combustor to transfer decomposed gases generated with the decomposition of ammonia to the combustor.