A heat treatment apparatus includes a processing container that accommodates a plurality of substrates, a gas supply unit that supplies a raw material gas into the processing container, an exhaust unit that exhausts the raw material gas in the processing container, and a heating unit that heats the plurality of substrates. The gas supply unit includes a gas supply pipe including: a first straight pipe portion that extends upward along a longitudinal direction of an inner wall surface of the processing container; a bent portion where a distal end side that extends above the first straight pipe portion is bent downward; a second straight pipe portion that extends downward from the bent portion; and a plurality of gas ejecting holes formed on the second straight pipe portion. The first straight pipe portion has a larger cross-sectional area than the second straight pipe portion.