In this paper, the liquid concentration sensor using crystal oscillator circuit which is available for measuring microdroplet is reported. We optimized the design of the sensing capacitor (SC) for sensing microdroplet. An electric-field-concentrated SC was used for measuring the liquid. The SC incorporated into the quartz oscillator has high-sensitivity because of the high stable crystal oscillator circuit. The SC is embedded in a silicone rubber. A spatially confined electric field is generated over the rubber. Therefore, the frequency of the oscillator changes by the contact with a liquid to the rubber. Moreover, the change less affected by a quantity of liquid. Because the frequency changes relate to the electrical characteristics of the liquid, the concentration of the liquid can be measured by the sensor. This sensor is expected to be applied to factory automation.