Provided is an apparatus for optical emission spectroscopy. The apparatus for the optical emission spectroscopy includes a light collection unit configured to collect light within a plasma process chamber in which plasma is generated to process a substrate, a light transmission unit configured to transmit the collected light, and an analysis unit configured to analyze the light provided through the light transmission unit, thereby analyzing a plasma state. The light collection unit includes a light collection part configured to concentrate the light generated in the plasma process chamber and provide the concentrated light to the light transmission unit.