PURPOSE: To reduce the quantity of dust sticking on wafers by a method wherein a means to slowly open a valve of a mass flow rate controller of a process gas is used.

CONSTITUTION: After carrying wafers 4 onto a lower electrode 3 in a vacuum processing chamber 1, the voltage VS proportional to the specified flow rate of a process gas is inputted to a processing circuit 10 from a flow rate setting up unit 11 so that the additional rate of this output VV may not exceed the follow-up capacity of a mass flow controller 5. By processing such a processing circuit 10 to slowly boost the voltage impressed upon the mass flow controller 5, the process gas can be progressively and slowly led into the vacuum processing chamber 1, thereby enabling the voltage to be boosted up to the specified value without overshooting at all. Through these procedures, any dust can be restrained from swirling to reduce the quantity of dust sticking on the wafers 4.