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This paper describes the development of a custom LSI which works at less than 0.5 μW for wireless sensor nodes used in health monitoring systems. To monitor body temperature and activity of animal or human with low power consumption at any time, ultra-low power signal processing are essential as well as low power sensors. We have deve loped measurement methods with bimetal type and piezoelectric type MEMS sensors which need almost no electrical power, and on-chip circuits of multistep selectable voltage reference generator for comparators to detect the output signal of the sensors. The piezoelectric sensor which generates even 1 mV of output voltage can be utilized by th is method. The LSI consists of CMOS combination logic circuits and works with 1 kHz RC oscillator to decrease the power consumption. This LSI including a wireless communication function could realize a wireless sensor node operating for more than 2 years using about 0.5 g of a 1.55 V button battery.