A method and apparatus for measuring the quantity of intake air of an internal combustion engine in its suction stroke include the operating steps of: heating a thermal resistor element located in the intake air path for a certain duration in synchronism with the suction stroke, detecting the electrical signal, which represents the resistance variation of the resistor element, at the end of heating and at the beginning of next heating, obtaining a value, which corresponds to the temperature variation of the resistor element, from the detected two electrical signals, obtaining the heat conduction factor of air from the obtained value, and obtaining a quantity of intake air from the obtained heat conductor factor of air.