PURPOSE: To clean an etching chamber without returning it to the atmosphere by a method wherein oxygen gas is introduced into the chamber and changed to plasma ions and a means to remove a contamination which has adhered to its inside wall is installed.

CONSTITUTION: First, a semiconductor substrate on which a mask has been formed is housed in an etching chamber 5 from a load-lock chamber 4 by using a conveyance manipulator installed in a separator chamber 3; it is plasma-etched at a high frequency; and a metal film on the substrate is etched selectively. Then, it is confirmed that the substrate has been treated; and the treated substrate is returned again to the lock chamber 4 by using the manipulator. After that, other substrates are transferred again to the chamber 5 from the lock chamber 4; and they are treated. When the treatment number has reached the prescribed number of wafers, the chamber 5 is set to an empty state and is evacuated until a prescribed degree of vacuum is obtained. After that, a cutoff valve 9 is opened; the flow rate of oxygen gas is instructed to a valve 8 by using a mass-flow controller 7; the gas is fed into the chamber 5; a plasma is generated at a high frequency; the gas is changed to ions; and a metal which has adhered is removed.