a heat generation type flow sensor of an enhanced output sensitivity which is destined for use , for example , as an air flow sensor employed in an engine control system of a motor vehicle . the heat generation type flow sensor includes a silicon substrate , a diaphragm disposed on the silicon substrate and having a cavity formed in a surface thereof , a flow rate detecting element provided on the diaphragm and including a heat generating resistor for outputting an electric signal indicative of a heating current flowing through the heat generating resistor , a supporting member for supporting the flow rate detecting element on the diaphragm in such a deposition that one surface of the diaphragm is exposed to a fluid for measurement while the fluid for measurement is difficult to flow into the cavity formed in the other surface of the diaphragm , and a control unit for performing such control that temperature of the heat generating resistor is sustained higher by a predetermined temperature than that of the fluid for measurement , wherein the heat generating resistor and the diaphragm are so dimensioned that ratio of a width of the heat generating resistor to that of the diaphragm is in a range from 0 . 4 to 0 . 6 inclusive and that ratio of a length in a longitudinal direction of the heat generating resistor to that of the diaphragm is in a range from 0 . 4 to 0 . 6 inclusive .