PURPOSE: To provide pretreatment for removing oxide completely from the surface of nickel plated on a wiring board when it is subjected to solder plating or solder leveling during production of printed wiring board.

CONSTITUTION: When the surface of nickel plated on a wiring board is subjected to solder plating or solder leveling during production of printed wiring board, the surface of nickel is treated with a mixture liquid of hydrogen peroxide and sulfuric acid. Since a clean nickel surface is obtained, wettability is improved in solder plating or solder leveling and a uniform thickness is obtained. Furthermore, adhesion of solder is enhanced, nickel surface is prevented from exposing at the time of mounting a component, deterioration due to oxidation of nickel is eliminated, and reliability is enhanced.