PURPOSE: To prevent short water and raise hot water temperature safely, easily and at low cost by completely insulating and jointing a radiator having a built-in resistor and a water level sensitive switch using a small float from and to each other, and submerging the radiator and the switch in hot water for water supply.

CONSTITUTION: The external surface of a concrete radiator with both of an electrical resistor and a heat resistant wire buried is coated with an external material 20 made of waterproofing and insulating heat resistant rubber, thereby forming an electrical shock preventive heater (BH). The case 13 of a water level sensitive switch (WS) having a float 10 guided with a guide rail 14 and freely movable vertically is fixed to the top of the BH with a screw 16. An electric cable 5 connected to the resistor via the switch (WA) is pulled out through the sealing cover 18 of the case 13 and connected to a plug 15 via a waterproofing switch 17, a thermal fuse 7 and a timer switch 8. According to this construction, the heater (BH) can be submerged in hot water, and secured with a suction cup 9 for the supply of commercial power, thereby preventing no-load operation and easily raising hot water temperature at low cost.