PURPOSE: To provide the device for desorbing a gas adsorbed in a vacuum vessel by an electric discharge only with the vacuum treating device evacuated to a high vacuum and easy to apply.

CONSTITUTION: A vacuum vessel 1 is evacuated by a vacuum pump 2 to a specified pressure, and a specified amt. of a gas supplied from a plasma producing gas feed part 6. When the vessel 1 is controlled to an appropriate pressure to produce plasma, a discharge power source 4 is started to start discharge. The discharge state monitoring signals thereafter are calculated by a process controller 5 to control the discharge and to determine when finishing degassing. Consequently, the discharge state is stably maintained, the time to finish degassing is appropriately determined, and the device is conveniently applied.