Abstract: A vertical type once-through heat recovery steam generator (HRSG) capable of improving operational stability during rapid startup and lifetime of a steam turbine while reducing environmental pollution caused by emissions, and a combined power generation system including the HRSG are provided. The HRSG includes a medium-pressure section including a medium-pressure side desuperheater installed at the rear of a medium-pressure superheater to lower the temperature of the steam supplied to a medium-pressure steam turbine, and a high-pressure section including a high-pressure side desuperheater installed at the rear of a high-pressure superheater to lower the temperature of the steam supplied to a high-pressure steam turbine.