Disclosed is an apparatus for optical emission spectroscopy which includes a light measuring unit measuring light in a process chamber performing a plasma process on a substrate, a light analyzing unit receiving light collected from the light measuring unit to analyze a plasma state, a control unit receiving an output signal of the light analyzing unit to process the output signal, and a light collecting controller disposed between the process chamber and the light measuring unit so as to be combined with the light measuring unit. The light collecting controller controls the light collected to the light measuring unit.