To prevent the occurrence of particles in a medium not heated when a furnace port cover 11 is moved downward and to prevent a reaction tube 4 from being damaged when outer force is given to an elastic body 203.

In the furnace, a screw 202 is tightened through the elastic body 203 so as to fix it in a state where a flat flange 5 protrusively formed at the outer periphery of the reaction tube 4 is sandwiched by a heater installation stand 1 on which the heater 2 is placed and a supporting member 201, and the unheated medium carried into the reaction tube 4 is heated by the heater 2. The furnace is provided with a positioning means 301 for deciding the distance between a supporting member 201 and the heater installation stand 1 when the unheated medium in the reaction tube 4 is carried outside or when a furnace main body is not used.