To easily release blockage, of downflow path even when the down flow path is blocked by providing an upward hole and a downward hole passing from outside to the downflow path and connecting supply pipes for compressed air to them.

Two upward holes 3 and 4 leading to a downflow path 2 is arranged so as to have the same diameter as the downflow path 2 and a shape horizontal between the outside and the middle point and upward between that point and the downflow path 2, opening by ca. 120 degrees to each other. A downward hole 5 is provided so that the height of the inlet to the downflow path 2 is the same level of the upward hole 4. To these upward and downward holes 3 to 5, compressed air supply pipes 13a to 13c are connected with fixing devices 8a to 8c. When the downflow path 2 is blocked, compressed air is introduced in the upward hole 3 to blow off blocking materials 35a upward the hole 3. If it is still blocked, the blocking materials 35b upward the upward hole 4 is blown off in the same manner. If it is not yet released, the blocking materials 35c is blown off downward the downflow nozzle with compressed air introduced in the downward hole 5 and the blockage of the downflow path 2 is released.