HBr and Cl.sub.2 are used as etching gases and Ar is used as a carrier gas in an ECR etching apparatus in which a semiconductor wafer is processed. Light emitted from plasma generated is dispersed by first and second spectroscopes to detect intensities of those spectra of the plasma which have first and second wavelengths. Both of these spectra are selected from those of an Ar atom. A CPU compares a present value, which represents a ratio of the spectral intensities detected, with a selected value of the ratio previously stored, and adjusts the intensity of a magnetic field such that the present value becomes closer to the selected value. The adjustment of the magnetic field intensity is carried out by changing the value of a current applied to magnetic coils. The magnetic field intensity is a parameter for adjusting an electron temperature of the plasma, and thus, the electron temperature of the plasma is adjusted by adjusting the magnetic field intensity.