An air flow measuring instrument, comprising: an auxiliary passage 8 arranged inside a main passage through which fluid flows, a tabular member 5 on which a pattern of a heating resistor for measuring an air flow is provided on one face 5a, the tabular member being disposed inside the auxiliary passage so that the one face 5a on which the heating resistor pattern of the tabular member is provided is disposed along a flow of fluid inside the auxiliary passage 8, a heating resistor pattern-side fluid passage 8a portion formed so that the fluid flows between the face 5a and a passage-forming surface 8d of the auxiliary passage, and a back-surface 8b side fluid passage portion formed so that fluid flows between a face 5b on a side opposite to the face of the tabular member and the passage-forming surface of the auxiliary passage. Guidance portion 13 guiding dust that collides against the end portion to back-surface side fluid passage portion 8b side is provided on upstream-side end of tabular member.