To prevent the surface temperature of a heater from undergoing dispersion and the heater from overheating with a simple construction and low cost by connecting a heat sensitive wire melted at above a fixed temperature for shorting a pair of conductors to an outside test terminal.

A heat sensitive wire 16 is fixedly wound around a heat generating part of a heat 12 immersed in a water tank 14 and the other end of the heat sensitive wire 16 has the inside conductor 22 and outside conductor 24 connected to the outside test terminal 19 of a leak detector body 1. For example, if the water tank 14 is turned down or broken by an earthquake or the like to expose the heater 12 to air, the temperature control by a controller 10 is disabled to overheat the heater 12 by continuous electrification. When the surface temperature of the heater 12 rises over about 180°C, a macromolecular heat sensitive body inside the heat sensitive wire abruptly melts and the conductor 22 makes a contact with the conductor 24, so that the terminal 9 short- circuits. Thus, current flows to a zero phase current transformer 2 and a control circuit 3 electrifies an exciting coil 4 to open contacts 5 and interrupt an electric path to a controller body 10 for stopping the electrification of the heater 12.