PURPOSE: To easily remove acid and alkali left in a through-hole in a wiring board by disposing a printed wiring board between electrodes placed in pure water and applying voltage between both electrodes.

CONSTITUTION: A pair of electrode plates 10, 10 are placed parallely in pure water 9 put in a cleaning tank 8 using a rack 11, and then a printed wiring board 12 is placed parallely between both electrode plates 10, 10 using a rack 11. Thereafter, DC voltage is applied between the electrodes 10, 10 to ionize acid (CaNO4, etc.) and alkali (NaCl, etc.) left in the through-hole 12a in the printed wiring board 12 into positive ions (Ca2+, Na+, etc.) and negative ions (SO42-, Cl-, etc.), which are in turn moved to the negative electrode 10 and to the positive electrode 10.