PURPOSE: To efficiently practice clog release by automatically detecting a clog, when caused in a pumping tube, and switching to positive rotation after reverse rotation.

CONSTITUTION: When a pumping tube is clogged by a foreign material such as concrete mass during normal operation, and positive rotating force feed can not be performed, the abnormality is detected from a pressure rise to a hydraulic motor 4 by a pressure switch PS, and the solenoid SOL1 of a solenoid valve 14 and the solenoid SOL3 of a motor switching valve 19 are demagnetized to stop concrete force feed and positive rotation of stirring. Solenoids SOL2 and SOL3 are excited to change over the hydraulic motors 4, 13 to reverse rotation. Thus, the concrete in the pumping tube is returned to a hopper, and flowability is recovered by kneading. After a determined time, the solenoids SOL1 and SOL are excited to change over the hydraulic motors 4 and 13 to positive rotation, and normal operation is returned. Thus, clog release can be automatically performed with high efficiency.