To prevent a heat retaining heater from being heated in empty by conducting a temperature of the heat retaining heater to the outside through a washing tank wall by a heat conduction plate and operating a thermal fuse by the heat conducted through the heat conduction plate in the outside of a washing tank to turn off the supply of electric power to the heat retaining heater forcedly.

A thermostat 20 is arranged on an external wall face in a bottom part of a washing tank to turn on and off the supply of electric power to a heat retaining heater 13 through a relay 22 due to contact opening and closing at a predetermined temperature for control. Three phase commercial power is supplied to the heat retaining heater 13 through open and close contacts of a temperature fuse means 25 and the relay 22 via earth breakage breakers 24 inserted into each phase in series. The temperature fuse means 25 is provided with a heat conduction plate means 26 conducting a temperature to the outside through a washing tank bottom part member 5a to conduct the temperature of the heat retaining heater 13. One phase power among three phase power passing through the earth breakage part breaker 24 is supplied to a control part 21 to use it as a power supply for the operation of an electronic circuit and supply a drive current to a coil of the relay 22 through opening/closing contact of the thermostat 20.