PURPOSE: To prevent dust particles from adhering to a wafer again while the wafer is being treated with a chemical liquid by installing the following: a dust counter which measures the number of dust particles in the chemical liquid; a detection means; and a pump control device.

CONSTITUTION: A wafer carrier 9 which houses wafers 10 is lowered; it is passed between a light source 7 and a photosensor 8. Then, the sensor 8 detects the wafer carrier 9 and is actuated; it actuates a pump control device 6, increases the number of revolutions of a pump 5 and increases the flow rate of a chemical liquid. When the carrier 9 is immersed in the chemical liquid in this state, the chemical liquid is overflowed into an outer tank 2 from the upper part of an inner tank 1. Then, the number of dust particles in the overflowed chemical liquid is measured by using a dust counter 3. After confirming that the number of dust particles has been reduced to the prescribed number or lower, the number of revolutions of the pump is reduced, and the flow rate of the chemical liquid is set to a nonoperating state. Thereby, it is possible to prevent that the dust particles coming from the wafers 10 adhere again and to enhance the economical efficiency and the reliability of the title apparatus.