High-sensitive heat detection sensor with a microfluidic chip is proposed, fabricated and evaluated, and also heat detection of a brown fat cell (BFC) has been demonstrated. The measurement principle relies on resonant thermal sensor, where the resonant frequency of a Si resonator changes due to incident heat to the resonator from a sample in water via a heat guide. The surrounding of the resonant thermal sensor is evacuated in vacuum area for minimizing the heat loss. The fabricated sensor has 1.6 mK of the temperature resolution, and 6.2 pJ of the detectable heat. As the result of the thermal measurement of the single BFC with stimulation, continuous heat production is observed. We show that this thermally insulated resonant thermal sensor has a high sensitivity, and is applicable to the observation of the thermogenic activity of a cell.