To provide an apparatus for preventing boil dry of heater for a water tank, which effectively prevents the boil dry of a heater 41 in spite of being a small size and inexpensive.

When a refractile body 52 is exposed from underwater to air, a light from a light-emitting device 55 is totally reflected at a boundary plane 52b and 52c and is received on a light-receiving element 57. At this time, an energization control element 19 is controlled by a control means 63, energization in energization lines 42 and 43 are intercepted to prevent the boil dry of the heater 41. At this time, since a movable body such as a float is unnecessary, the apparatus is easily miniaturized and even if dust and contaminant are contained in water in a water tank 11, they have no influence on a light path, the boil dry of heater is surely detected.