Provided is a high-precision, mechanically robust thermal humidity sensor. A detecting element 1 of the thermal humidity sensor of the present invention has a diaphragm (a bridge structure) 2 formed on a planar substrate which is formed from a material with high thermal conductivity such as silicon or ceramic. Formed on the diaphragm 2 are temperature detecting resistors 4, 5, 6, and 7 and a heating resistor 3 arranged in a manner surrounding the temperature detecting resistors. Humidity is detected based on the outputs of the temperature detecting resistors 4, 5, 6, and 7. Accordingly, humidity measurement errors that can possibly occur due to the leakage of heat through the diaphragm 2 to the planar substrate can be reduced.