An improvement is proposed in the cleaning treatment of semiconductor silicon wafers in which the conventional step of cleaning with an aqueous solution of an acid is replaced with a cleaning treatment with a temporarily acidic pure water which is produced electrolytically by the application of a DC voltage between an anode and a cathode bonded to the surfaces of a hydrogen-ion exchange membrane so that the acidic cleaning treatment can be performed under mild conditions so as to eliminate the troubles unavoidable in the conventional process. The apparatus used therefor comprises a rectangular vessel partitioned into a central anode compartment, in which the wafers are held in a vertical disposition within an upflow of pure water, and a pair of cathode compartments on both sides of the anode compartment by partitioning with a pair of hydrogen-ion exchange membranes, on both sides of which an anode plate and a cathode plate are bonded.