PURPOSE: To eliminate defective printing or missing of circuit by cleaning the printing face of a board with ozonic water of specified concentration and then drying the board.

CONSTITUTION: Prior to printing with ink, silk screen, etching resist, and the like, a printed wiring board is cleaned for 0.5-180sec with ozonic water of concentration in the range of 0.2-7mg/l and then it is dried. Concentration below 0.2mg/l causes incomplete cleaning whereas concentration above 180mg/l does not enhance cleaning effect. Cleaning may be performed through spraying or immersion and treating time is preferably set at 0.5-10sec for spraying whereas 2-180sec for immersion. No specification is set for drying. This method removes organic substances adhered to the printing face while decomposing into water or carbon dioxide and suppresses missing of circuit.