PURPOSE: To prevent the freezing of an electrolytic cell without building in a heater for overheating water by using a power source transformer combinedly as the heater.

CONSTITUTION: This device comprises an electrolytic cell 2 with an electrode 3 built in a casing 1 for electrolysing water, a power source for applying the voltage to the electrode 3 of the clectrolytic cell 2 and a temperature sensing means 4 for sensing the temperature in the casing 1. When the temperature in the casing 1 is judged to be lower than the set temperature by the temperature sensing means 4, power is applied to the primary side in the state of cutting off the secondary side of a power source transformer 5 of the power source from the electrode 3, and the inside of the case 1 is heated by utilizing the heat development generated by no-load loss of the power source transformer 5 to prevent the freezing of the electrolytic cell 2.