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1. An airflow measuring device comprising:
a housing having a bypass passage, which is configured to receive a part of air flowing through a duct when the housing is mounted to the duct, and a sub-bypass passage, which is branched at a branch from an intermediate portion of the bypass passage and configured to receive a part of air flowing through the bypass passage; and a flow rate sensor located in the sub-bypass passage and configured to detect a flow rate of air flowing through the sub-bypass passage, whereinthe branch from the bypass passage into the sub-bypass passage is configured to separate dust contained in air flowing through the duct and to flow the dust into the bypass passage, and
a passage wall surface defining the bypass passage on an upstream side from the branch includes:
a first wall surface located on a side where the sub-bypass passage branches, the first wall surface curving to direct a flow, which is along the first wall surface toward a downstream, to be away from the branch; anda second wall surface opposed to the first wall surface, the second wall surface curving to be away from the branch at least in a portion opposed to the branch.
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