PURPOSE: To maintain the initial characteristics of a humidity detecting element all the time, by conducting a current through a heating member for heat cleaning of the humidity detecting element by the delayed output of a starting signal.

CONSTITUTION: When an ignition switch SW1 is closed and a transistor Tr1 is turned ON, a Tr2 is turned OFF, a Tr3 is turned ON, a heater RH is conducted for a preset time period T, and a humidity detecting element SH is heated. When the element SH reaches a steady state and an AC signal is applied from an oscillator circuit 1, a voltage corresponding to humidity is obtained at an output terminal.