To provide a balcony submergence alarm device capable of detecting early the occurrence of submergence of a balcony caused by the plugging of a water collecting port or the like.

The balcony submergence alarm device 10 comprises a submergence detecting sensor 12 disposed in a position higher than the water collecting port 7 of the balcony 1 and lower than the highest part of a floor face 6 of the balcony 1, and an alarm 13 emitting an alarm sound and alarm light when the submergence detecting sensor 12 detects submergence. The submergence detecting sensor 12 is disposed at a stepped part 14 between a water collecting ditch 8 and the floor face 6 and between a backing material 3a forming the floor face 6, and a surface material 5 covering the backing material 3a, 3b. Further, an electric wire 15 for transmitting a detection signal output from the submergence detecting sensor 12, to the alarm 13 is wired between the backing material 3a, 3b and the surface material 5.