are provided to obtain an accurate dimension after trimming based on an amount of roughness of a mask edge or an amount of radicals in plasma a wafer on the surface of which a desirably patterned mask for etching is formed is processed in a plasma etching process chamber and the mask is trim treated by plasma etching so as to reduce the patterned mask width the apparatus includes a plasma monitor for measuring an amount of radicals in the plasma process chamber and a trimming condition calculator for calculating a time required for the trimming to obtain a desired mask width based on a pre measured width dimension of the patterned mask and a pre measured amount of roughness of a mask edge and the radical amount measured by the plasma monitor the trimming is performed for the time calculated by the trimming condition calculator application_number US 79021204 A description preferred embodiments of the invention will now be described with reference to the accompanying drawings referring first to fig1 the system construction of an etching apparatus 12 having a trimming treatment function will be described in fig1 high frequency power generated in a high frequency power supply 100 is fed to an antenna 102 by way of a high frequency.