A rubidium oscillator or a cesium oscillator is used as a high stability oscillator, and an OCXO being a metastable oscillator which is inferior in a long-term frequency stability compared with the above oscillators but has a high short-term frequency stability is used as a backup. There is prepared a table in which an elapsed time since an occurrence of an abnormality in the high stability oscillator and weighting (use ratio) of use of the both oscillators is corresponded, and by using this table, after the high stability oscillator recovers, an oscillation frequency of the metastable oscillator is used by 100% initially, but thereafter the weighting (use ratio) of use of the metastable oscillator is made smaller and the use ratio of the high stability oscillator is made larger in stages.