An airflow-rate detecting device capable of detecting humidity has a housing, a flow rate sensor, a humidity sensor, a humidity sensor case, and a dust separating portion. The housing therein defines a bypass passage into which a part of air flowing in a duct flows. The flow rate sensor is disposed in the bypass passage. The humidity sensor detects a humidity of air flowing in the duct at an outside of the housing. The humidity sensor case houses the humidity sensor and therein defines an interior space into which a part of air flowing in the duct flows as a target air of which humidity is detected by the humidity sensor. The dust separating portion removes dust from the target air before the target air flows into the interior space.