A humidity sensor formed of a sintered compact consisting essentially of material substantially of a spinel structure represented by the formula: M.sub.1-x A.sub.x Fe.sub.2 O.sub.4-.alpha. where M stands for magnesium or zinc, A stands for an alkali metal, x is the numerical value in the range of 0.001 to 0.2, and .alpha. is the number of oxygen vacancies. The humidity sensor is highly sensitive over a wide relative humidity range, withstands a long use and presents almost no hysteresis in its humidity-resistance characteristics.