An atomic oscillator includes a gas cell that has metal atoms sealed therein, a heating unit that heats the gas cell, a heat transmission unit that is positioned between the gas cell and the heating unit, is thermally connected to the gas cell, and transmits heat generated by the heating unit to the gas cell, and a light absorbing unit that is thermally connected to the gas cell so as to be separated from the heat transmission unit and absorbs heat of the gas cell. The heat transmission unit includes a gas cell accommodation portion including at least a pair of gas cell accommodation walls disposed outside the gas cell, and a thermal conductive elastic member which is interposed in a gap formed by the gas cell and the gas cell accommodation walls of the heat transmission unit.