An air flow rate measuring device for measuring intake air flow rate in an automotive engine, improved to reduce any error attributable to variation in the configurations of air cleaner and/or duct forming air passage upstream of the flow rate measuring device, thus offering high accuracy of the flow rate measurement. In one aspect of the invention, the inlet opening of a sub-air passage is disposed to contain the axis of a main air passage, while the outlet opening of the sub-air passage is disposed in the vicinity of a wall defining the main air passage. In order to cause a local reduction of the pressure of the principal of the air flow at the outlet opening, a protrusion or a flat surface which causes impediment to the flow if air in the main air passage is provided immediately upstream of the outlet opening. In another aspect, the inlet surface in which the inlet of the sub-air passage opens is stepped in the direction of flow of the air so as to enable air to be introduced into air flow rate measuring portion in the sub-air passage from different regions in the main air passage while giving a preselected order of preference to these different regions.