A thermal air flow sensor that offers high flow rate measurement accuracy is provided. The thermal air flow sensor includes a measuring element. The measuring element includes: a semiconductor substrate; a heating resistor and a temperature measuring resistor both formed as a result of thin films being stacked over the semiconductor substrate; an electronic insulator including a silicon oxide film; and a diaphragm portion formed after part of the semiconductor substrate is removed. The heating resistor and the temperature measuring resistor are formed over the diaphragm portion. In the thermal air flow sensor, a ratio of an area occupied by the thin films to an area of the measuring element ranges between 40% and 60%.