PURPOSE: To maintain the temperature of hot water substantially at a predetermined value by determining the temperature of water within a copper boiler by a set temperature of a temperature ajusting meter, and automatically starting and stopping an electrically driven pump.

CONSTITUTION: When a gas burner 2 is ignited and a buckwheat noodle oven 1 is heated, water within a copper boiler 4 is heated by means of the heat transfer surface 3 of a water cooling water and the flue heat transfer surface 3 below a chimney 9. When the temperature of water within the copper boiler 4 increases to the preset temperature of the temperature regulating meter 10 in which the preset temperature can be arbitrarily varied, the electrically driven pump 4 is started by the operation of the contact of the temperature regulator 10, and hot water within the copper boiler 4 is sent to a hot water storage tank 15 via a pipeline 14. During the ordinary operation of the device, hot water in the hot water storage tank 15 is maintained at a substantially predetermined temperature set by the regulator 10.