Monitoring of biosignals plays important roles in health management in daily life. Especially, the sensing method of biosignals, which does not require special efforts such as restraining people or wearing the sensors to measure them, is really important to maintain monitoring activities of people. We call such method what a sensing way should be for monitoring of biosignals as casual sensing methods. Previously, we have developed highly sensitive and wide-measurement-range load sensors based on measuring frequency shift of quartz crystal resonator (QCR) as the sensing principal. We have integrated the load sensor into a chair, and measure the load when people just sit on it. Since the load sensor has wide measurement range of 105 order, multi-biosignal; heartbeat, respiration, and body motion under the weight-loaded environment can be measured from load information. Thus, people can casually monitor the biosignals by sitting on the chair. However, the previous sensing system required us to measure the frequency shift of one-Hz order in tens-MHz of the resonant frequency of a QCR. In this case, the sensing system requires a expensive frequency counter, and it is not suitable for daily use situation such as inhome sensing. In this paper, we presents the detection of multi-biosignal method using a newly developed load sensing system which utilizes the developed frequency counter unit. In order to measure the frequency shift of one-Hz order in tens-MHz of the resonant frequency, we use a differential method of signal for two QCRs with an electrical signal-mixing circuit. By using the method, we can reduce the required measurement range for frequency from tens-MHz to tens-kHz. The load sensing performances were evaluated, and the results showed that the sensitivity and withstand load were 2.9 [mN] and 300 [N], respectively. Finally, we demonstrated the measurement of multibiosignal by using constructed system, and succeeded in detecting respiration, heartbeat, and body motion.