THERMAL FLOW SENSOR

The invention relates to a thermal flow sensor which is fabricated at a low cost and has improved reliability. Over a cavity (7) formed in a semiconductor substrate (2), at least a heating resistance (4) is formed near the center of the cavity with an electrical insulation film interposed between the heating resistance and the cavity. The temperature (Th) of the heating resistance (4) is controlled to be higher than the medium temperature (Ta) by a constant temperature (”Th = Th - Ta). A distance (Ws) in the direction of airflow from an upstream end of the heating resistance (4) to an upstream end of the electrical insulation film lying over the cavity and the constant temperature (”Th) satisfy the following relationship: ”Th / Ws ‰¤ 800 °C / mm Thus, a thermal flow sensor is provided which can prevent deposition of floating fine particles, such as carbon particles, caused by the thermophoretic effect, can be fabricated at a low cost, and has high reliability.