A humidity measuring device configured to measure humidity of a gas, the humidity measuring device comprising: a measurement unit including a temperature sensing element configured to measure temperature, and a humidity sensing element configured to measure humidity; a casing including an accommodating section in which the measurement unit is accommodated; and a display unit including a display device, and which is fixed to the casing, characterized in that the gas at the measurement location where the humidity of the gas is measured is the gas that is discharged from a dehumidifying device configured to dehumidify the gas; and the display unit further comprises: a display control unit configured to control the display device to thereby cause the display device to display at least one of the humidity of the gas measured by the humidity sensing element, or a dew point temperature of the gas calculated based on both the temperature of the gas measured by the temperature sensing element and the humidity of the gas measured by the humidity sensing element; the display unit comprises a replacement determination unit configured to determine whether or not the dehumidifying device has been replaced; and in a case that the humidity of the gas measured by the humidity sensing element is higher than a threshold value immediately after having determined that the dehumidifying device has been replaced, the display control unit controls the display unit to thereby cause a display to be displayed on the display device, the display prompting the user to replace the measurement unit. a measurement unit including a temperature sensing element configured to measure temperature, and a humidity sensing element configured to measure humidity; a casing including an accommodating section in which the measurement unit is accommodated; and a display unit including a display device, and which is fixed to the casing, characterized in that the gas at the measurement location where the humidity of the gas is measured is the gas that is discharged from a dehumidifying device configured to dehumidify the gas; and the display unit further comprises: a display control unit configured to control the display device to thereby cause the display device to display at least one of the humidity of the gas measured by the humidity sensing element, or a dew point temperature of the gas calculated based on both the temperature of the gas measured by the temperature sensing element and the humidity of the gas measured by the humidity sensing element; the display unit comprises a replacement determination unit configured to determine whether or not the dehumidifying device has been replaced; and in a case that the humidity of the gas measured by the humidity sensing element is higher than a threshold value immediately after having determined that the dehumidifying device has been replaced, the display control unit controls the display unit to thereby cause a display to be displayed on the display device, the display prompting the user to replace the measurement unit. a display control unit configured to control the display device to thereby cause the display device to display at least one of the humidity of the gas measured by the humidity sensing element, or a dew point temperature of the gas calculated based on both the temperature of the gas measured by the temperature sensing element and the humidity of the gas measured by the humidity sensing element; the display unit comprises a replacement determination unit configured to determine whether or not the dehumidifying device has been replaced; and in a case that the humidity of the gas measured by the humidity sensing element is higher than a threshold value immediately after having determined that the dehumidifying device has been replaced, the display control unit controls the display unit to thereby cause a display to be displayed on the display device, the display prompting the user to replace the measurement unit.