PURPOSE: To provide a detecting device having superior sensitivity and durability by using two kinds of silane compounds represented in specific formulae, heteropolyacid compound, an organic solvent, and a heteropolyacid compound contg. solid comprising water contg. acid at a specific rate.

CONSTITUTION: A humidity detecting device uses (a) A mixture mixed silane compounds expressed by formula (I), formula (II) with a mol ratio of 80:20-20:80; (b) at least one heteropolyacid compd. selected from the group comprising a phosphomolybdic acid hydrate, a phosphomolybdate hydrate, a phosphotungstic acid hydrate, a phosphotungstate hydrate, a silicomolybdic acid hydrate, a silicomolybdate hydrate, a silicotungstic acid hydrate, and a silicotungstate hydrate; (c) an organic solvent; and (d) a heteropolyacid compound contg. solid comprising water contg. an acid with a rate of 0.001-5wt.%. The mol ratio of a to b to c to d is 1:0.001-0.5:10-100:1-30. Heat treatment at 40-550 degrees C is applied.