To provide a fault protecting circuit for a heater element in a conduit for a patient concerning a humidifying system for the respiratory organs.

The fault protecting circuit includes a spark detector similar to an overcurrent detector. The spark detector has some forms which include a spark detector formed of a transformer 2, 5 having two coils. Once the change of a current exceeding some level is detected or a voltage level exceeds a threshold, the protecting circuit interrupts current flowing through a heater element 3 for a previously set period.