To provide an inlet door of a heat treatment chamber in a vacuum carburizing furnace with a simple and inexpensive structure where only elevating/lowering means for the door is present as a driving means for the door, and the other driving means is not particularly required.

The inlet door of a heat treatment chamber comprises: an intermediate plate 12 in which seal members 9, 9 are arranged at both the sides in such a manner that the opening part 6 at the front face of a cooling chamber 2 as a heat treatment chamber to be charged with a work 5; and two outside inlet door 1a and inside inlet door 1b sandwiching the intermediate plate 12 and arranged so as to be contacted with both the sides of the seal members 9, 9. The outside inlet door 1a and inside inlet door 1b are guided by direct-acting bearings 14, 14, and can be elevated/lowered individually. The intermediate plate 12 has a communication hole 23, and when the outside inlet door 1a and the inside inlet door 1b has descended and come to the state of shielding the inner circumferential space 24 of the intermediate plate is made, the inside of the inner circumferential space 24 of the intermediate plate is exhausted via the communication hole 23 of the intermediate plate 12, and the outside inlet door 1a and the inside inlet door 1b are pressed against the intermediate plate 12 by the atmospheric pressure at the outside of the cooling chamber and the internal pressure of the cooling chamber 2.