Abstract: In a heat exchanger of the present invention for exchanging heat between fluid flowing in a flow passage in a heat exchanger tube and fluid flowing outside of said heat exchanger tube, a solid bar-like insertion member or a hollow bar-like insertion member whose opposite ends are closed is provided in said flow passage in which a fluid having phase change flows in gas-liquid two phase state or liquid phase state, a cross section of said insertion member is formed into a substantially circle shape, a polygonal shape or a starlike shape, and a cross-sectional area of a flow passage in which said fluid flows is reduced as a mass flow rate quality of said fluid is reduced. With this construction, it is possible to restrain the pressure loss at the time of evaporation, and the evaporation ability can be enhanced or can be restrained from being deteriorated.