A semiconductor device manufacturing method, including: mounting substrates on a mounting table within a processing chamber along a rotation direction of the table; starting to supply a first-element-containing gas to a first region in the chamber along the rotation direction, while rotating the table and exhausting the processing chamber; starting to supply a second-element-containing gas to a second region in the chamber; starting to generate, by a plasma generating unit in the second region, plasma of the second-element-containing gas in the second region to have a first activity; and forming a thin film containing first and second elements on the substrates by rotating the table to cause the substrates to sequentially pass through the first and second regions in turn so that a first-element-containing layer is formed in the first region and is modified in the second region by generating plasma having a second activity higher than the first activity.