A condenser comprising: a high pressure side condenser; a high pressure side cooling tube bank provided inside the high pressure side condenser, which has a high pressure side cooling water introduced therein and condenses a high pressure side turbine exhaust by heat exchange with the high pressure side cooling water to obtain a high pressure side condensate; a high pressure side hot well provided at a bottom portion of the high pressure side condenser; a low pressure side condenser which has an inner pressure lower than the high pressure side condenser; a low pressure side cooling tube bank provided inside the low pressure side condenser, which has a low pressure side cooling water introduced therein and condenses a low pressure side turbine exhaust by heat exchange with the low pressure side cooling water to obtain a low pressure side condensate; a pressure shroud provided at a lower part than the low pressure side cooling tube bank, inside the low pressure side condenser; a low pressure side hot well provided at a lower part of the pressure shroud, of the low pressure side condenser; high pressure steam introducing means (10, 30) provided at the low pressure side hot well, for communicating with an inner side of the high pressure side condenser and introducing high pressure steam; low pressure side condensate introducing means provided at the pressure shroud, for introducing a low pressure side condensate into the low pressure side hot well, characterized by comprising a flash box (14, 24) which is arranged to flash a heater drain from a feed water heater to generate flash steam, and to introduce the generated flash steam into a lower side of the pressure shroud, in an interval between the low pressure side cooling tube bank and the low pressure side hot well. a high pressure side condenser; a high pressure side cooling tube bank provided inside the high pressure side condenser, which has a high pressure side cooling water introduced therein and condenses a high pressure side turbine exhaust by heat exchange with the high pressure side cooling water to obtain a high pressure side condensate; a high pressure side hot well provided at a bottom portion of the high pressure side condenser; a low pressure side condenser which has an inner pressure lower than the high pressure side condenser; a low pressure side cooling tube bank provided inside the low pressure side condenser, which has a low pressure side cooling water introduced therein and condenses a low pressure side turbine exhaust by heat exchange with the low pressure side cooling water to obtain a low pressure side condensate; a pressure shroud provided at a lower part than the low pressure side cooling tube bank, inside the low pressure side condenser; a low pressure side hot well provided at a lower part of the pressure shroud, of the low pressure side condenser; high pressure steam introducing means (10, 30) provided at the low pressure side hot well, for communicating with an inner side of the high pressure side condenser and introducing high pressure steam; low pressure side condensate introducing means provided at the pressure shroud, for introducing a low pressure side condensate into the low pressure side hot well, characterized by comprising a flash box (14, 24) which is arranged to flash a heater drain from a feed water heater to generate flash steam, and to introduce the generated flash steam into a lower side of the pressure shroud, in an interval between the low pressure side cooling tube bank and the low pressure side hot well.