A humidity measuring device for measuring humidity of intake air in an internal-combustion engine, the device characterized by comprising: a water droplet adhesion determination means for determining whether or not a water droplet adheres to a surface of a sensor element which detects the humidity, based on changes in humidity and temperature of the intake air, a relative humidity sensor, a temperature sensor for detecting the temperature of the intake air, a signal processor, wherein the water droplet adhesion determination means is configured to calculate a determination value based on a rate of change of the temperature of the intake air and a rate of change of relative humidity of the intake air, compare the determination value with a threshold, and determine that a water droplet adheres to the surface of the sensor element when a state where the determination value is larger than the threshold continues for a determination time or longer; and wherein the water droplet adhesion determination means is configured to determine that a water droplet adheres to the surface of the sensor element when the following condition is satisfied for a determination time or longer, THid < Id = Δ ⁢ T ∗ Kt Δ ⁢ RH ∗ Krh wherein ΔT represents the rate of change of the temperature, ΔRH represents the rate of change of the relative humidity, Id represents an index which is a determination value calculated based on the rate of change ΔT of the temperature and the rate of change ΔRH of the relative humidity, THid represents the threshold and Kt and Krh represent coefficients; wherein the coefficients Kt and Krh are determined from temperature and humidity in order to take into account the reaction speeds and the accuracy of the humidity sensor and the temperature sensor depending on the temperature and the humidity. a water droplet adhesion determination means for determining whether or not a water droplet adheres to a surface of a sensor element which detects the humidity, based on changes in humidity and temperature of the intake air, a relative humidity sensor, a temperature sensor for detecting the temperature of the intake air, a signal processor, wherein the water droplet adhesion determination means is configured to calculate a determination value based on a rate of change of the temperature of the intake air and a rate of change of relative humidity of the intake air, compare the determination value with a threshold, and determine that a water droplet adheres to the surface of the sensor element when a state where the determination value is larger than the threshold continues for a determination time or longer; and wherein the water droplet adhesion determination means is configured to determine that a water droplet adheres to the surface of the sensor element when the following condition is satisfied for a determination time or longer, THid < Id = Δ ⁢ T ∗ Kt Δ ⁢ RH ∗ Krh wherein ΔT represents the rate of change of the temperature, ΔRH represents the rate of change of the relative humidity, Id represents an index which is a determination value calculated based on the rate of change ΔT of the temperature and the rate of change ΔRH of the relative humidity, THid represents the threshold and Kt and Krh represent coefficients; wherein the coefficients Kt and Krh are determined from temperature and humidity in order to take into account the reaction speeds and the accuracy of the humidity sensor and the temperature sensor depending on the temperature and the humidity.