PURPOSE: To obtain linear frequency control characteristics by removing a frequency variable error caused by the digital synthetic ratio control in a frequency synthesizer in the digital control atom oscillator with an optical microwave resonator.

CONSTITUTION: The frequency synthetic ratio in a frequency synthesizer 4 is controlled by a control digital signal D. The error in the variable characteristics of output frequency f occurred at that time is corrected when the intensity of the magnetic field in an optical microwave resonator 1 is controlled. Therefore, a digital/digital converter 8 being composed of a memory having an address part corresponded to the control digital signal D is provided. The magnetic field in the resonator 1 is controlled by preliminarily storing the control digital data including the correction data in the memory 8 and by reading the data including the correction data from the memory 8 by the control digital signal D from outside.