A physical section of an atomic oscillator includes: a gas cell in which gaseous metal atoms are sealed, and the gas cell includes a first window having optical transparency; a light source that emits excitation light toward the metal atoms through the first window; a first heating unit that disposes at the first window and that is located between the first window and the light source; and a Peltier element that is stacked on the first heating unit, that is located between the first heating unit and the light source, and that decreases a temperature of a side of the Peltier element facing the light source than a temperature of an opposite side of the Peltier element facing the gas cell.