To obtain a diffusion furnace apparatus in which particles are not generated and which restrains the air from being mixed by providing a remaining installation cap which is used to prevent the air from being mixed from a gas exit at a reaction tube and which is constituted so as to be capable of being taken into, and out from, the reaction tube by using a fork.

A diffusion furnace apparatus is constituted mainly of a horizontal quartz reaction tube 20 and of a heater used to heat the reaction tube 20, and it is provided with a remaining installation cap 10 which is used to restrain the air from being mixed from a gas exit 7 as the open end of the reaction tube 20 and which is constituted so as to be capable of being taken into, and out from, the reaction tube 20 by using a fork. The remaining installation cap 10 comprises a rectangular or trapezoidal cutout at the lower part so that the square pillar part of the fork is inserted easily, and it is constituted of three disk-shaped quartz plates whose diameter is smaller than the inside diameter of the reaction tube 20 and of three quartz support rods which are arranged in parallel so as to support and fix the three quartz plates in such a way that their surfaces are faced.