This paper presents a concept of a micro thermal sensor to be used for defect inspection of a smoothly-finished surface such as a bare wafer or a hard disk. In the proposed concept, existences of the defects on a measurement surface will be detected by scanning the surface with the micro thermal sensor, which is utilized to detect the variation of the thermal flow in-between the sensor surface and the measurement surface. The proposed micro thermal sensor has a possibility of detecting various types of surface defects. An existence of the defect having a convex shape such as an asperity or a particle can be found by detecting a heat generated at the collision between the sensor surface and the tip of the defect. In addition, the thermal sensor is expected to be applied for the inspection of pits or scratches in a concave shape on the smooth surface. When the thermal sensor is placed with respect to the measurement surface with a tiny gap of less than 1 μm, a heat transfer system sensitive against the gap variation will be constructed at the interface between the thermal sensor and the measurement surface becomes a system.