system in the apparatus shown in fig1 the electrolyte is supplied to the overflow tanks and then cleaned so that attachment of particles or bubbles may be reduced to the porous surface of treated substrate enabling more uniform chemical treatment in the present invention a conductive bulkhead such as a wafer for preventing metal contamination may be provided between the treated substrate and the positive metal electrode in order to avoid direct contact between the electrolyte and the positive metal electrode in such an arrangement the metal is prevented from dissolving into the electrolyte thus preventing metal contamination on the treated substrate also an arrangement can be employed in the present invention that the hermetic contact between the treated substrate and the sealing member is achieved by a sealing member arranged obliquely to the main surface of treated substrate and urged against the peripheral portion thereof further the present invention permits one of the electrodes to be set on the back surface of the treated substrate in addition the treated substrate such as wafer can be effectively transported in the present invention using a cassette for carrying the treated substrate as shown in fig1 in fig1 a wafer cassette 108 is formed as a plane plate member in which an aperture 108a shaped to fit the contour of a wafer as the treated substrate is formed in the central portion a step 108b is formed on the lower portion of inner wall in the.