The present invention relates to an apparatus for detecting photons according to an atmospheric condition, using a function of adjusting light quantity that can significantly improve reliability of an atmospheric condition analysis result by minimizing noise in a spectrum by maintaining the quantity of incident light uniform within a predetermined range regardless of atmospheric conditions and changes, and to a method of adjusting light quantity. The apparatus for detecting photons in accordance with atmospheric conditions using a function of adjusting light quantity includes: an apparatus case having a light inlet; a light quantity adjuster disposed under the light inlet and adjusting quantity of incident light such that a predetermined quantity of light travels inside; and a controller controlling operation of the light quantity adjuster in accordance with intensity of light detected by the light quantity adjuster.