A moisture detector of closed case type according to a heat capacity method is realized by a relatively simple constitution. The moisture detector includes a first temperature sensor for sensing temperature of a detection target; a second temperature sensor for sensing the temperature of the detection target; a small heater for heating the second temperature sensor; and a processing unit for obtaining information relating to moisture percentage in the detection target on the basis of a signal outputted from the first temperature sensor and a signal outputted from the second temperature sensor heated by the small heater.