PURPOSE: To prevent the bottom of a reaction tube from being brought into contact with a pedestal and to facilitate the removal, replacement of the pedestal by so bringing the part of legs 1 of the pedestal into noncontact with the bottom of the tube that the bottom is separated from a pedestal body near the port of the tube.

CONSTITUTION: A pedestal 10 is inserted from a port 5 into the processing chamber 2 of a reaction tube 1, and placed on the bottom of the chamber 2. The pedestal 10 covers the bottom of the chamber 2 corresponding to a wafer mounting position from the vicinity of the port 5 substantially in contact with a cap 9 and slightly floats from the bottom by legs 11. In this case, the legs 11 are isolated from the bottom of the tube 1 in the vicinity 12 of an inlet in which reaction product tends to be mostly accumulated. A boat 7 is conveyed from the port 5 of the tube 1, and slid on the pedestal 10 to be set to a predetermined position of the chamber. Thereafter, the chamber 2 is heated by a heater 3 to a predetermined temperature, impurity gas generated from a diffusion impurity source disposed on an impurity holder 8 is brought into contact with wafer 6 group held on the boat to be diffused as predetermined.