Ballistocardiogram (BCG) is useful and informative signal result from the cardiac activity. In this paper, we present a practical BCG measurement system with bed sensors. Moreover, the simple algorithm to detect all individual heartbeats and estimate beat-to-beat interval lengths is presented. These are effective for all night continuous monitoring of cardiovascular system and enable us to detect sleep stage. Heart beats is detected by cross-correlation method. This method has three steps, preprocessing, typical wave form creation and cross-correlation calculation. Output of this process were validated with 10 subjects. As the results, the average 71.5% of the beat-to-beat intervals were detected. Moreover this study revealed that the individual difference is significant factor to detect heart beats precisely.