To provide a single chamber type vacuum heat treatment furnace and an oxidation preventing method of treatment item in the single chamber type vacuum heat treatment furnace capable of providing a favorable treatment item by effectively preventing dew formation in a short time, and improving work efficiency.

The single chamber type vacuum heat treatment furnace A is equipped with a furnace body 1 with a water-cooled jacket 15 provided on a furnace wall 11 and a treatment space S for the treatment item W formed in an interior by the furnace wall 11, heating equipment 2 heating the treatment item W placed in the furnace body 1, a cooling gas feeder 3 feeding cooling gas into the treatment space S, a first coolant circulating system cooling the treatment item W via the cooling gas by a heat exchanger 52 placed in the treatment space S, a pressure reducing device 4 reducing a pressure in the treatment space, and a second coolant circulating system feeding a coolant C to the water-cooled jacket 15. It is characterized by that the second coolant circulating system is equipped with a coolant heating part 65 heating the coolant C.