The present disclosure provides an air flow rate measuring device disposed in an intake air passage. The air flow rate measuring device measures a flow rate of the intake air flowing through the intake air passage. The air flow rate measuring device includes a casing, a flow rate sensor, and a sensor module. The casing defines a bypass passage to take in a portion of the intake air. The flow rate sensor is disposed in the bypass passage. The flow rate sensor generates an output signal according to a flow rate of the intake air. The sensor module protrudes from an outer wall of the casing. The sensor module includes a multi-sensor unit that has a relative humidity sensor and a temperature sensor. The relative humidity sensor is exposed to an inside of the intake air passage to detect a relative humidity of the intake air flowing through the intake air passage. The temperature sensor detects a temperature of the intake air flowing through the intake air passage.