Abstract: A vertical heat treatment apparatus includes a casing, a vertical heat treatment furnace provided in the casing, a substrate holding unit mounted in the casing for holding substrates to be heat-treated in the vertical heat treatment furnace, a loading/unloading unit having a wafer boat for supporting the substrates, the loading/unloading unit being adapted to put the substrates in and take the same out of the vertical heat treatment furnace, and a transportation robot for moving the substrates between the substrate holding unit and the wafer boat. The vertical heat treatment apparatus further includes a clean air supplying unit for supplying clean air sideways to the wafers supported by the wafer boat when the loading/unloading unit is at an unloading position, a and duct for introducing air from the outside of the apparatus. The clean air supplying unit is provided with an air filter disposed opposed to the wafer boat.