PURPOSE: To separate the layer of liquid contaminant and to rake out it from an aperture by a method in which while the flexible strip which rotates in the form rotating at high speed, is slid, it is inserted into a concrete tube.

CONSTITUTION: A rotary shaft 4 is held horizontally and is rotated around its central line (c) at high speed by a driving motor 19 through a chain 18. The long hole 20 parallel to the central line (c) is provided on the rotary shaft 4, and the flangelike fitment 7 fitted slidably onto the rotary shaft 4 in the direction of the central line (c), is provided. The key 21 projected to the inner periphery of the fitment 7 is inserted slidably into the long hole 20, and the tip of the key 21 is connected to the sliding tube 22 arranged rotatably in the rotary shaft 4, and then the position of the fitment 7 is regulated by sliding the sliding tube 22. A plurality of the flexible strips 8 such as a rubber or a chain are provided between the outer peripheral surface of a fitment 6 and the outer peripheral surface of the fitment 7 along the direction of a screw, whereby the central part 8' of the flexible strip 8 is made heavier than its both end parts, and the flexible strip is made apt to receive the action of centrifugal force.