An atomic oscillator that controls an oscillation frequency by using an optical absorption property derived from a quantum interference effect occurring when two kinds of resonance light are made incident as coherent light having different wavelengths from each other, includes an optical system that includes: a gas cell sealing metal atoms in a gas state therein; a coherent light source for supplying the resonance light to the metal atoms being in the gas cell; and a light detector for detecting light transmitted through the gas cell. In the atomic oscillator, a first refraction unit is formed at a light incident side, on which coherent light is made incident, of the gas cell.