PURPOSE: To identify dryness, dew formation and frosting by providing a plurality of sensor units with a pair of electrodes on the surface of a detection element having a smaller dielectric constant than ice in such a manner as to have the electrodes of each unit opposite to each other at a specified space.

CONSTITUTION: Ceramic plates 1 and 2 as detection element having a smaller dielectric constant than ice are prepared and a pair of comb-shaped opposed electrodes 3 and 4 are formed on the under-surface of the plate 1. Likewise, a pair of comb-shaped opposed electrode 5 and 6 are formed on the top of the plate 2. The plate 1 with the electrodes 3 and 4 formed is turned over in the direction as shown by the drawing and moved in the direction B to approach the plate 2. Then, the plates are opposed to each other with insulation spacers 7a and 7b at a predetermined space. Leads of the electrodes 3 and 6 are connected together to form one lead section 8a of an identifying sensor while leads of the electrodes 4 and 5 are connected together to form the other lead section 8b. Thus, the state of dryness, dew formation or frosting in the atomsphere of a gap 9 can be detected depending on the changes in the impedance between the lead sections 8a and 8b.