To improve the working efficiency of position adjustment and other maintenance performed on a fork in a horizontal diffusing furnace and, in addition, to solve the trouble of a quartz shutter caused by rotation.

The horizontal diffusing furnace 100 comprises a core pipe 101 having a horizontally opened throat 101a, a heater 102 which raises the internal temperature of the pipe 101, and a scavenger 104 which exhausts an unreacted gas leaking out of the pipe 101. The furnace 100 also comprises the quartz shutter 103 which opens and closes the throat 101a by sliding in parallel with a plane including the throat 101a. The shutter 103 is composed of first and second areas 103a and 103b, and the first area 103a has an opening having a size set so that maintenance may be performed on a product bringing in/out furnace or the inside of the core pipe 101. The shutter 103 closes the throat 101a by means of the second area 103b during the operation of the furnace 100, and by means of the first area 103a when maintenance is performed on the product bringing-in/out furnace or the inside of the core pipe 101.