To shorten a washing time.

A washing apparatus 30 is equipped with the space partitioning member 32 arranged so as to cover the stamper 16 fixed to a fixed plate 12. The space partitioning member 32 is internally evacuated to about 10-1 Torr by a vacuum pump 36 to be sucked to the fixed platen 12. The space partitioning member 32 receives the supply of oxygen gas from an oxygen gas source 40 and high frequency voltage is applied to the discharge electrode 42 arranged in the space partitioning member 32 by a high frequency power supply and gas discharge by oxygen gas is generated between the discharge electrode 42 and the earthed stamper 16. The oxygen gas supplied into the space partitioning member 32 becomes active species 46 such as oxygen plasma by discharge to collide with the stamper 16 and these active seeds react with the org. matter bonded to a cavity forming surface and the org. matter is decomposed to be gasified to wash the stamper.