PURPOSE: To reduce local friction in a tube by providing a tube whose inner diameter is gradually reduced from the suction side towards the exhaust side and increasing the pressure in the closed section of the tube, thus reducing any reverse flow of concrete.

CONSTITUTION: When preceding roller 7 starts to move away from tube 3 opening said tube, the suction (closed section) and exhaust sides partitioned by said preceding roller 7 acting as the boundary are communicated with each other. As the suction side is at a higher pressure, the reverse flow of the exhaust side concrete to the suction side is reduced. As said reverse flow of concrete cannot be stopped completely, local friction develops to some extent on the inner face of the tube 3 on exhaust side between A and C. As thickness 3' of this part has been increased and the eccentricity of the center of rotation O' of rotor body 4 is H, the compression at thicker part 3' of tube 3 is large and even if local friction develops, roller 7, 7' can close the opening passage of tube 3 completely.