PURPOSE: To oxidize deposit in a short time without damage, moisture absorption of a heat insulator by plasma discharging at a low temperature in an atmosphere containing oxyger in a vacuum furnace shell.

CONSTITUTION: Process gas containing oxygen is introduced from process gas supply means into a vacuum furnace shell 1. Then, a current is supplied from a plasma power source 13 to electrodes 12, and a plasma discharge is conducted at a low temperature in an atmosphere containing oxygen. A space in the shell 1 becomes a plasma atmosphere by the plasma discharge. Further, since a heat insulator 2a has gas permeability, a processing space 3 in a treating chamber 2 also becomes the plasma atmosphere. Deposit is rapidly oxidized to eliminate activity by the plasma atmosphere. When the deposit is oxidized, the plasma discharge is stopped, the shell 1 is returned to the atmospheric pressure, opened with the atmosphere, and an object maintenance operation is executed. Thus, the maintenance operation can be safely conducted.