To provide a heat exchanger which secures pressure resistance enabling the use of a high-pressure gas refrigerant and can be manufactured at drastically reduced cost and is superior in heat efficiency, in a heat exchanger of such constitution that it exchanges heat between the primary side, where a refrigerant circulate and the secondary side where the water of fluid to be heated circulates.

The heat exchanger is manufactured, using a double pipe 30 for heat exchange, where a cylindrical refrigerant circulation pipe 10 having many refrigerant circulation holes 11 operated to pass through a wall is made by aluminum extrusion processing and a water pass pipe 20 is inserted into the refrigerant circulation pipe 10, prior to pipe expansion processing thus joining the refrigerant circulation pipe 10 around the water passage pipe 20.