A thermal type air flow measuring instrument is mounted to a main passage (5) forming an intake air passage of an internal combustion engine and is provided with an auxiliary passage (3) which includes a heating resistor (1) and a temperature sensing resistor (2) therein and an electronic circuit (8) which is electrically connected to the heating resistor and the temperature sensing resistor. The auxiliary passage is a passage including at least one bent portion (303) composed of a first path (302) substantially parallel to the main passage and a second path (302) having a path different in flow direction from the first path. A member (4) forming the auxiliary passage is fixed in a united form to only a case member for interiorly protecting the electronic circuit, and the case member is fixed to the outer wall surface of the intake air passage. The auxiliary passage structure capable of being handled as a single module united to the case member is effective to the mass production of an air flow control system and the reduction in cost thereof since the structure can be mounted to different kinds of internal combustion engines.