PURPOSE:To provide an effective deceleration of high speed flow of steam fed into a condensor other than discharged gas of turbine represented by a turbine bypass steam in reference to the condensor mounted in a nuclear power plant and to prevent the high speed flow from being against the discharging gas of the turbine. CONSTITUTION:A decelerator device 30 is composed of row of columns 31 arranged perpendicular to the flow of steam, and an arrangement of the row of columns is made such that a ratio between a diameter of the column and a pitch of the columns is about 2-10. A space of the decelerator device 30 is set more than five times of the diameter of the column so as to cause the steam flow to be decelerated down to an acceptable limit speed of a cooling pipe in respect to the inlet speed of the steam flow. In thrn, in a plane just over a group of cooling pipes 14 and positioned downstream of the steam inlet port 17 is arranged a deceleration promoting device 34 composed of a row of columns 33 so as to be oppositely arranged against the discharging flow of turbine 32, the row of columns 33 is arranged in a circular area 35 near the wall 16b against which the steam flow fed from the steam inlet port 17 is abutted such that a ratio between a pitch of columns and a diameter of solumn is of about 1.1-3.0 in closely spaced-apart relation. And, in other area 36, a ratio between a pitch of the column and a diameter of the column is about 2.0-5.0 to form a coarse spaced-apart relation.