since side walls facing each other in a direction perpendicular to the direction of a main flow are present at an inflow port into the diversion passage, a portion of the flow having an angle inclined) relative to the parallel flow of the main flow is obstructed by the side walls and is prevented from flowing into the diversion passage. Thus, increased flow rate sensitivity due to increased flow velocity inside the diversion passage becomes minimal. Consequently, the flow velocity inside the diversion passage is slow, leading to a decrease in the flow rate sensitivity and making output of the flow rate signal unstable. 
Since the side walls facing each other in the direction perpendicular to the direction of the main flow have a predetermined thickness in adirection perpendicular to the direction of flow of the main flow, the flow of the main flow colliding with the side walls at right angles is interrupted temporarily by the side walls, giving rise to a delay before flowing into the diversion passage. Consequently, the state of the flow into the diversion passage is unstable, making output of the flow rate signal unstable. 
In air flow rate measuring apparatuses for internal combustion engines, foreign matter such as droplets of liquid (water, oil, etc.), dust etc., contained in the air flowing through the airflow channel is blown and introduced into the diversion passage from the upstream end. When this foreign matter such as droplets of liquid, dust, etc., adheres to inner wall surfaces of the diversion passage or the