PURPOSE: To detect the circulating state of a treating soln. and to surely circulate the soln. by placing a tank in which a fixed quantity of a treating soln. is stored only during normal circulation and by monitoring the surface of the soln. in the tank.

CONSTITUTION: A treating soln. 12 fed to an outer tank 2 through a valve 1 is circulated to an inner tank 4 with a circulation pump 7 through a filter 8 to treat a body 10 to be treated. The soln. 12 in the inner tank 4 overflows into a buffer tank 3 having a drain hole 5 and the soln. in the tank 3 overflows into the outer tank 2. The size of the drain hole 5 is regulated so that the amount of the treating soln. circulated by overflow into the buffer tank 3 is made larger than the amount of the soln. flowing out of the hole 5. The surface of the soln. in the buffer tank 3 is sensed with a sensor 6 for detecting a circulating state and a shortage of the amount of the treating soln. circulated or the stop of circulation is detected by the lowering of the surface of the soln. in the tank 3.