PURPOSE: To generate water flow even in a corner part and to prevent settling and sticking of cement by providing a driving part on a rack gear and providing an underwater agitating mixer in the vicinity of the lower end of a spur gear.

CONSTITUTION: A driving part having a spur gear 2 engaged with a rack gear 1 is provided on the approach to the side edge upper than the water surface of a sludge tank 5. The main shaft 3 of the spur gear is vertically hung underwater and an underwater agitating mixer 4 is provided in the vicinity of the lower end. An ejection port is provided on the outside from the direction of installation and fitted with a projection part 9 provided on the rack gear. A cam rod 11 projecting from a worm reduction gear 10 is engaged with a turning arm. Reciprocating movement caused by reducing the speed of the cam rod with the reduction gear is transmitted to a turning arm 7 via the cam rod 11. An underwater agitating mixer 4 is freely turned by sliding the rack gear 1. Thereby working efficiency can be prevented from being lowered.