To enable mounting of a highly fluid concrete-placing device even on the lower opening of an existing hopper by providing an attachment for attaching a pliable tube having a flat discharge port to the lower opening of a hopper.

When the placement of highly fluid concrete is stopped to the mold flame, the flat discharge port 21 of a rubber tube 20 of a hopper 10 is strongly pressed at both sides through the closing positional movement of left and right openable rollers 40, 40. Accordingly, the flat discharge port 21 of the rubber tube 20 is kept to a positive closed state without any leakage. In the case when handling of high fluid concrete is not required, and it is required to remove a highly fluid concrete-placing port device 14, the attachment part 30 is removed from the lower opening 13 of the hopper 10 by loosening bolts and the like, and the support bracket 43 is dislodged from the hydraulic cylinder 50.