A semiconductor device manufacturing method includes a plasma etching step for etching an etching target film formed on a substrate accommodated in a processing chamber. In the plasma etching step, a processing gas including a gaseous mixture containing predetermined gases is supplied into the processing chamber, and a cycle including a first step in which a flow rate of at least one of the predetermined gases is set to a first value during a first time period and a second step in which the flow rate thereof is set to a second value that is different from the first value during a second time period is repeated consecutively at least three times without removing a plasma. The first time period and the second time period are set to about 1 to 15 seconds.