We propose a new method for obtaining narrow-band electrocardiographic potential (ECG) and respiratory variation in infants through their underwear. This method is based on capacitive coupling involving the electrode, the underwear, and the skin. In order to detect ECG signal and respiratory variation individually, a differential separation filter is introduced. The filter divides an input signal into a high frequency component for narrow-band ECG detection and a low frequency component for respiratory variation detection. To evaluate the proposed method, a pilot measuring device was made and tested. As the results, we confirmed the following: (1) Respiratory variation signal obtained with our device was highly correlated (r = 0.995) with that measured simultaneously with a commercial pneumotachograph. (2) Our pilot device succeeded in measuring narrow-band ECG and respiratory variation respectively in 13 of 15 and 11 of 15 measurements of infant aging 53 to 187 days. (3) R-R interval extracted from the narrow-band ECG was 25 times more precise than that calculated from photoplethysmogram. (4) Heart rate variability extracted from the narrow-band ECG was 100 times more precise than that calculated from photoplethysmogram. These results reveal that the proposed method appears promising for application to bedding of infants and neonate as a non-invasive and awareness-free approach for ECG and respiratory variation monitoring during sleep.