The opening 52 is a gas exhaust port formed so as to be capable of exhausting the gas in the inner tube 44. The length of the opening 52 is formed to be equal to the length of the wafer boat 38 or to extend upwards and downwards in the vertical direction to be longer than the length of the wafer boat 38. That is, the upper end of the opening 52 extends to be located at a height equal to or higher than a position corresponding to the upper end of the wafer boat 38, and the lower end of the opening 52 extends to be located at a height equal to or lower than a position corresponding to the lower end of the wafer boat 38. Specifically, as illustrated in FIG. 2, the distance L2 in the height direction between the upper end of the wafer boat 38 and the upper end of the opening 52 is in the range of about 0 mm to 5 mm. In addition, the distance L3 in the height direction between the lower end of the wafer boat 38 and the lower end of the opening 52 is in the range of about 0 mm to 350 mm.

The lower end of the reaction tube 34 is supported by a cylindrical manifold 54 formed of, for example, stainless steel. A flange part 56 is formed at the upper end of the manifold 54, and the lower end of the outer tube 46 is installed and supported on the flange part 56. A seal member 58 such as an O-ring is interposed between the flange part 56 and the lower end of the outer tube 46 so that the inside of the outer tube 46 is hermetically sealed.