PURPOSE: To output a signal having a very highly stable frequency by controlling a resonance frequency of a cavity resonator depending on the calculated resonance frequency.

CONSTITUTION: An electromagnetic coupling means 22 is coupled with a cavity resonator 6 electromagnetically so as to apply a measuring signal from a measuring signal oscillator 21 to the cavity resonator 6 in a hydrogen maser device thereby extracting the measuring signal from the cavity resonator 6. A detection means 23 detects the resonance frequency in the electromagnetic field mode of the cavity resonator 6, that is, the 2nd resonance frequency from the measuring signal outputted from the electromagnetic coupling means 22 to obtain an accurate value of the 2nd resonance frequency f'c. A calculation means 24 calculates the resonance frequency fc of the electromagnetic field mode in the cavity resonator 6 corresponding to the standard frequency fm from the accurate value of the 2nd resonance frequency f,c obtained in such a way. A control means 25 controls the resonance frequency fc of the cavity resonator 6 to a prescribed value depending on the calculated resonance frequency fc.