An outlet of a sub passage that returns measured gas, which has passed through a flow sensor, from the sub passage to a main passage opens on an outer wall of a housing toward a downstream side in a reference direction. The outer wall of the housing includes a protrusion on the downstream side of the outlet. When the outlet and the protrusion are projected onto a projection plane perpendicular to the reference direction, the outlet and the protrusion partly overlap with each other on the projection plane. A relationship of θ1