To provide a rubidium atomic oscillator, wherein the generated heat from the oscillator self is drastically reduced so as to be able to use a constant temperature bath attached crystal oscillator on market the specification of a high temperature side of which is 70°C MAX.

The rubidium atomic oscillator includes: a constant temperature bath attached piezoelectric oscillator 2; a frequency control circuit 3; an atomic resonator 10; a first external shield case 4 for surrounding the constant temperature bath attached piezoelectric oscillator; and a second external shield case 11 for surrounding the atomic resonator, wherein the atomic resonator includes: a cavity part; a first internal shield case 25 for surrounding the cavity part; a lamp part; and a second internal shield case 35 for surrounding the lamp part, and light transmittance sections 25a, 35a are respectively formed to the first and second internal shield cases equivalent to an optical path between a light emitting section of the lamp part and a light receiving section of the cavity part.