Despite the adverse cardiovascular consequences of sleep apnea syndrome (SAS), the majority of patients remain undiagnosed. Recently, the ECG-based detection method for diagnosing sleep disordered breathing (SDB) was proposed, where SDB is evaluated by detection of cyclic variation of heart rate (CVHR) associated with sleep apnea/hypopnea. However, this method cannot apply to ECG for atrial fibrillation (AF). To test the possibility the ECG-based detection for SDB in AF patients, we study the method using R-peak amplitude and heart rate variability (HRV). We estimated the power spectrum of the R-peak amplitude and HRV time series in 24 patients with both AF and SAS, and then calculated very low frequency (VLF) powers associated with CVHR. To assess predictive accuracy regarding detection of severe SAS (AHI > 30 /h), AUC of ROC analysis was calculated. The fraction of VLF power of the R-peak amplitude time series showed the largest AUC (= 0.73).