A humidity detector a humidity sensor detecting a relative humidity of an air inside a sensor case housing the humidity sensor. The humidity detector has an air volume obtaining section, a flow direction obtaining section, a setting section, and a correction section. The air volume obtaining section obtains air volume information correlated with an air volume of the air flowing around the humidity sensor. The flow direction obtaining section obtains flow direction information correlated with a flow direction of the air flowing around the humidity sensor. The setting section sets correction factors configuring a dynamic compensator based on the air volume information and the flow direction information. The dynamic compensator compensates for a response delay of the humidity sensor. The correction section corrects a detection value, which is detected by the humidity sensor, by using the dynamic compensator to obtain the relative humidity of the air.