An optical system of an atomic oscillator that regulates an oscillation frequency by using an optical absorption property by one of a double resonance method utilizing light and micro waves and a coherent population trapping (CPT) method utilizing a quantum interference effect produced by two kinds of resonance light, includes: a light source emitting the resonance light; a gas cell disposed at an emitting side of the light source, sealing a gaseous metal atom therein and transmitting the resonance light through a metal atom gas; a light detecting unit detecting the transmitted light that is transmitted through the metal atom gas; and a fluorescence blocking unit blocking at least a part of fluorescence, which is emitted from the metal atom gas to the light detecting unit, and disposed between the metal atom gas and the light detecting unit.