Abstract: A humidity determining device 20 includes first and second humidity determining sensors 17, 18 and a control unit 21. When the first and second humidity determining sensors 17, 18 react with a chemical substance, the humidity determining performance of the first humidity determining sensor 17 and the humidity determining performance of the second humidity determining sensor 18 change with time at different amounts of change. The control unit 21 computes a difference value between a first humidity value and a second humidity value determined by the first and second humidity determining sensors 17, 18. When the difference value is greater than a predetermined reference value, the control unit 21 outputs an anomaly signal indicating that the humidity determining performance of at least one of the first or second humidity determining sensors 17, 18 has been reduced.