The invention relates to a thermal-type air flow measuring instrument comprising: temperature sensing resistors (1a, 1b) for detecting the flow rate provided within an auxiliary passage (3) provided within a fluid passage (5), wherein the length L of the auxiliary passage along the forward flow through the auxiliary passage is larger than the length ℓ of the auxiliary passage (3) along the forward flow through the main passage (5), and a passageway (309) for directing a reverse flow toward the temperature sensing resistors (1a, 1b) is provided in the auxiliary passage (3).