Provided is an apparatus configured to measure radical spatial density distribution including a process chamber including a viewport, a driving device configured to move a moving wall inside the process chamber, a light source configured to generate light, a collimator disposed in the viewport of the process chamber and configured to transmit light received from the light source to the moving wall and receive light reflected from the moving wall, and a spectrometer configured to receive the reflected light from the collimator, and measure radical spatial density based on analyzing an absorption amount of a spectrum of the received light.