Heat treatment apparatus and method of manufacturing semiconductor device

A heat treatment apparatus capable of achieving high-accuracy processing and high safety and a method of manufacturing a substrate are provided. The heat treatment apparatus 10 includes a reaction tube 42 for processing a substrate, a manifold 44 for supporting the reaction tube 42, a heater 46 installed around the reaction tube 42 to heat an inner part of the reaction tube 42, a surrounding member 500 installed to surround a side portion of the reaction tube 42 arranged in a lower portion than the heater 46; an exhaust device 301 for forcibly exhausting a gap 506 between the surrounding member 500 and the reaction tube 42; and a sealing member 150 installed in a contacting portion between the reaction tube 42 and the manifold 44. Here, an inlet hole 501 through which the exhaust device inhales an atmosphere outside the surrounding member 500 to the gap 506 is installed in the surrounding member 500.