A semiconductor device manufacturing method comprises a process of forming a film on each of multiple substrates arrayed in a processing chamber by a thermal CVD method by supplying a film forming gas into the processing chamber while heating the interior of the processing chamber, wherein in the film forming process, a cycle is performed one time or multiple times with one cycle including a step of flowing the film forming gas from one end towards the other end along the substrate array direction, and a step of flowing the film forming gas from the other end towards the one end along the substrate array direction, without forming temperature gradient along the substrate array direction in the processing chamber.