PURPOSE: To prevent the consumption of an extra amount of energy in a cycle of vertical movement of porous disks.

CONSTITUTION: An apparatus is equipped with a tank 2, lots of porous disks installed in the tank 2, a shaft 4 which holds the porous disks, a hydraulic cylinder 43 which is installed on the top of the tank 2 and linked with the shaft 4 to move the shaft 4 vertically, and a controller which controls the pressure of oil supplied to the oil chamber of the hydraulic cylinder 43. Since constant force can always be applied to the shaft 4, the consumption of an extra amount of energy in a cycle of vertical movement of the porous disks can be prevented, causing neither lowering the durability of a reciprocating jet reactor nor generating noise. Moreover, since a driving device 41 is simplified and the equipment mounted on the tank is reduced, the load can be decreased.