Disclosed is a double pipe heat exchanger for a boiler supplying heating water and hot water, which can prevent water from being condensed even if cold heating water or cold water is introduced into an inlet of the heat exchanger in the boiler for supplying heating water and hot water The heat exchanger includes: a plurality of outer heating water pipes to which combustion heat is applied from a burner of a combustion chamber; a plurality of inner pipes, each of which is inserted in each outer pipe; a returned heating water pipe in which cold heating water is introduced; and a cold water pipe in which cold water is introduced, wherein the returned heating water pipe and the cold water pipe are connected to one of the inner pipes. The heat exchanger has a structure in that returned cold heating water or cold water is firstly introduced into the inner pipe of an heat exchanger, sufficiently preheated, and introduced into the outer pipe of the heat exchanger again, thereby preventing water from being condensed around an inlet of the heat exchanger through which returned cold heating water or cold water is introduced.