PURPOSE: To prevent an erroneous operation and to positively detect only when the water drops by detecting an intermittence of an alternating signal from a water drop sensor, and changing said intermittence to a water drop signal, thereby judging that the water drops when the water drop signal reaches a preset number.

CONSTITUTION: An alternating signal E2 output when between conductive bodies L1,L2 of a water drop sensor 2 is short-circuited by water is changed to a rectangular pulse signal E3 by a conversion circuit 3. A water drop signal circuit 4 detects the intermittence of the pulse signal E3, and changes the signal to a water drop signal E4 by a multivibrator 4a. A water drop judging circuit 5 judges that the water drops when the water drop signal E4 reaches a preset number set by a counting number setting device 14, with outputting a water drop judging signal CAR from a preset counter 6. An alarming circuit 15, in response to the signal CAR, generates an alarm. Accordingly, in this structure, the water drop sensor is prevented from erroneously operated due to the generation of dew drops on the surface of a water pipe etc., making it possible to positively detect only the drop of water resulting from the actual breakage of the water pipe etc.