PURPOSE: To provide a heat exchanger that has a high heat exchanging efficiency and is economical by changing the fin pitch in the axial direction of a heat transmission pipe so that where the fluid flowing outside the tube is in the gaseous phase, the pitch is made small, and where the fluid is in the liquid phase, the pitch is made large.

CONSTITUTION: Where there is more gaseous refrigerant among the total refrigerant, the layout is made such that an area of smaller fin pitch formed by fins 4a contacts the gaseous refrigerant so that the heat conducting surface per unit length of the heat transmission pipe may be large to enhance the heat exchanging efficiency. As the gaseous refrigerant is condensed and the condensed liquid quantity increases, the fin pitch formed by fins 4b is made large so that the condensate 5 collected by the fins may easily be removed from the surface of the heat transmission pipe. Since the condensation process of the gaseous refrigerant advances as the gaseous refrigerant flows from the area where the fin pitch is small through an annular passage 6 including the fin groove 3 outside the pipe, the refrigerant contacting more condensed liquid shifts toward the area where the fin pitch is larger. Therefore, the condensed liquid film on the fins become thinner, resulting in prevention of the reduction in the refrigerant area and the overall heat conducting coefficient.