PURPOSE: To use a heating medium working at a higher boiling point and at a lower melting point than water and carry more quantities of heat than water at a constant temperature in a wide range from cold temperature up to high temperature and convert the water into cold water and the hot water into steam by means of a heat exchanger.

CONSTITUTION: An end side of a single heating medium circulation flow passage 6 is connected with a heating and cooling side of a heat exchanger 4 installed in a water supply pipeline 3 to a discharge section where a heat source 7 and a cold heat source 7 are selectively connected with a start end side of the circulation flow passage 6 by way of a three-way valve 10. There is further encapsulated ethylene glycol, which is a heating medium (a) at a higher boiling point and a lower melting point than water so as to feed higher heat from the cold heat to the discharge section 2 by way of the circulation flow passage 6.