A semiconductor device manufacture method includes the steps of forming a resist layer above a work target layer; exposing and developing the resist layer to form resist patterns including isolated pattern and dense patterns; monitoring widths of isolated and dense pattern of the resist patterns to determine trimming amounts of linewidths to be reduced; determining etching conditions for realizing the trimming amounts of both the isolated and dense patterns, the etching conditions using mixed gas of a gas having a function of mainly enhancing etching and a gas having a function of mainly suppressing etching; trimming the resist pattern under said determined etching conditions; and etching the work target layer by using said trimmed resist patterns. A desired pattern width an be realized stably by trimming using plasma etching.