PURPOSE: To detect the replacement period of a humidity sensing material by generating a on-off signal by a cleaning heater arranged nearby the humidity sensing material and counting one-shot pulses generated according to the output signal.

CONSTITUTION: The humidity sensing material 1 deteriorates in sensitivity owing to the sticking of matte in an atmosphere, so the humidity sensing material 1 is heated by a heater 2 and cleaned. At this time,a cleaning signal generation part 8 generates a on signal for turning on the heater 2. This on signal allows a one-shot pulse, which is counted by a pulse counter 10. Further, the set number of times of heat cleaning which indicates the replacement period of the humidity sensing material 11 is set in a pulse setting part 11. Then, a digital comparator 12 compares the value of the pulse counter 10 with the value of a setting counter 11 and generates a warning 13 of the replacement period of the humidity sensing material 1 when the pulse counter value increases above the set counter value. Thus, the replacement period of the humidity sensing material is detected by the simple circuit constitution.