PURPOSE: To prevent a contaminant from being removed insufficiently from a substrate by a method wherein substrates are housed in a cassette in such a way that the surface faces the surface of adjacent substrates and that the back faces the back of the substrates and an electrode is arranged between the substrate and the substrate.

CONSTITUTION: A plurality of substrates 2a, 2b,... 2i which have been housed in a cassette 4 are immersed in the inside of a cleaning tank 1. At this time, in the case of the adjacent substrates, e.g. 2a and 2b as well as 2b and 2c, the surface 2aS of the substrate 2a faces the surface 2bS of the substrate 2b, the back 2bB of the substrate 2b faces the back 2cB of the substrate 2c, and the substrates are housed in the cassette 4. Electrodes 3a, 3b,... 3j are arranged respectively in such a way that they are situated on face sides of the individual substrates. When a changeover switch 6 is set to the side of a contact 6a, a positive potential is applied to the electrodes 3a, 3c,... 3i from a DC power supply 7a, and a negative potential is applied to the electrodes 3b, 3d,... 3j. Thereby, a current does not flow between the electrode and the electrode during a cleaning operation, and a potential difference is reduced at end parts of the electrodes.