To provide a heat treatment furnace which reduces the number of times of washing a quartz core tube to improve productivity and prevents contamination on a semiconductor wafer during heat treatment.

The thermal treatment furnace comprises a furnace body 3 which is heated by a heating means 2 and receives process gas G from the outside, a quartz core tube 5 which is stored in the furnace body 3 and has a thermal treatment instrument 4 therein for filling a heated substance, and a cover 8 which is detachably mounted on a part of an inner surface 6 of the quartz core tube 5 and is made of quartz glass, silicon, or hyperpure silicon carbide.