To rapidly start a plant without raising the concentration of the dissolved oxygen in condensate at the time of start of DSS of the plant.

A condenser comprises a tube bundle chamber 3 for cooling and condensing the exhaust air of a steam turbine and housing a tube bundle, a hot well chamber 4 partitioned from the tube bundle chamber 3, a connecting pipe 7 having an openable isolation valve 6 through which the chamber 3 is connected to the chamber 4, and a deaerating chamber 11 communicating with the tube bundle chamber 3 through an opening part. Steam supply pipes 22 and 24 are respectively opened to a condensate residence part 9 in the tube bundle chamber 3 and to the deaerating chamber 11.