To provide a preventive method of involvement of oxygen gas in a diffusion system for preventing the involvement of oxygen gas into a diffusion furnace (furnace tube) and monitoring the concentration of oxygen in the diffu sion furnace, and to provide a suitable diffusion furnace in this method and a fork system.

In the diffusion system 1A, each inert gas feeding tube 115 is assembled in an upper shutter 113a and a lower shutter 114a of a furnace tube 110, and the inert gas is spouted from a spouting opening 116 formed at the inner face thereof and forming a curtain (C) when the shutters 113a and 114a are opened. The fork system 50A has a constitution at a mounting part 512 so that an upward inert gas shower can be spouted in an expanded state sideways from branched nozzles 532a, 532b, and 532c of the inert gas feeding tube 530.