PURPOSE: To clean a part mounted substrate with argon gas plasma while preventing the oxidation of a copper pattern formed on the substrate surface.

CONSTITUTION: Argon gas is introduced into a vacuum chamber 10, a high-frequency high voltaage is impressed between the electrodes 16 and 18 provided in the chamber 10 to convert the argon gas into plasma, the plasma is allowed to impinge on the surface of the electronic part mounted substrate 22 placed on the electrode 16 connected to a high-frequency power source 20, and the substrate 22 surface is cleaned. In this case, 5-10vol.% of hydrogen gas is introduced into the argon gas to convert the hydrogen gas into plasma along with the argon gas to clean the substrate 22 surface.