A semiconductor device manufacturing method including a step of forming, by thermal chemical vapor deposition, silicon nitride films on a plurality of substrates vertically stacked in a vertical reaction tube having an inner wall. Bis tertiary butyl amino silane and NH.sub.3 flows into the vertical reaction tube and flows vertically from one end of the plurality of substrates to an opposing end of the plurality of substrates without flowing into the vertical reaction tube through the inner wall at a height between the one end and the opposing end of the plurality of substrates. The silicon nitride films are formed on the plurality of substrates in a state in which a distance "a" between adjacent substrates of the plurality of substrates and a distance "b" between edges of the plurality of substrates and the inner wall of the vertical reaction tube are maintained substantially equal to each other.