PURPOSE: To allow the membrane resistor of a flow sensor element to be capable of withstanding a repeated use state, by crystallizing the membrane resistor and adjusting a crystal grain boundary to a predetermined size.

CONSTITUTION: In a membrane type air flow amount apparatus constituted by a method wherein the flow sensor element used in the constant temp. type heat generation air flowmeter of a fuel supply system for supply fuel fitted to the sucking air flow amount of an internal combustion engine is formed in a membrane form and overcoated with glass, the above-mentioned membrane resistor is crystallized and the crystal grain boundary thereof is adjusted to a size of 1W5μm.