PURPOSE: To create a uniform ascending flow of chemical in an overflow bath without creating eddy current and maintain and improve the cleanliness of the liquid in the bath by a method wherein a liquid transmitting film is attached near the bottom of the bath main part facing the bottom and the liquid is supplied into a space defined by the bottom and the liquid transmitting film.

CONSTITUTION: If a chemical with high cleanliness is supplied into a space 7 under a liquid transmitting film 3 is a bath from a supply inlet 2, because of the liquid transmitting film 3, a differential pressure is induced between the liquid above the film 3 and the liquid below the film 3 corresponding to the transmission loss. The pressure in the space 7 can be considered to the approximately constant and the differential pressure is almost uniform over the whole surface of the liquid transmitting film 3. as the quantity of the liquid transmitted through the liquid transmitting film 3 is determined by the differential pressure, a uniform ascending flow is created over the whole surface of the liquid transmitting film 3. Although partial eddy currents of standing liquid may be induced if jigs such as a basket which are necessary for processing are put in the bath, eddy currents over the whole bath are not created and the chemical which is in the bath and whose cleanliness is degraded can be substituted by the liquid with high cleanliness efficiently.