To prevent electric discharge from being generated between a heater and a furnace wall and satisfactorily heat treat an article to be treated by supplying argon gas to a maffle through which the article passes, providing the heater in a space independent from the maffle located between the furnace wall and the maffle, and supplying nitrogen gas to the space.

A cylindrical graphite maffle 2 is provided in the longitudinal direction of a furnace chamber constructed with a furnace wall 1 comprising a carbon or graphite heat insulating material. The maffle 2 and the furnace chamber are encapsulated with a sealing member 6 independently from each other. An electric resistor heater 10 is provided between the furnace wall 1 and the maffle 2. Argon gas is supplied to the maffle 2 through an argon gas supply pipe 7 for protecting an article 4 to be heated in the maffle 2, and nitrogen gas is supplied to a space 13 between the furnace wall 1 and the maffle 2 through a nitrogen gas supply pipe 9 for electrically protecting the electric resistor heater 10 and protecting the furnace wall 1.