The combined cycle power system includes a fuel cell, a gas turbine, a fuel gas supply line, a compressed air supply line, a discharged fuel recirculation line, a discharged air supply line, a gas turbine fuel supply line, a discharged fuel atmospheric release line, a discharged air atmospheric release line, and a discharged fuel release bypass line in which a bypass on-off valve is provided in a flow passage that connects the discharged fuel atmospheric release line and the discharged fuel recirculation line downstream of the discharged fuel release source valve and downstream of the discharged fuel circulation source valve.