Abstract: The heat exchanger performance of a stack type heat exchanger used as an aircraft engine oil cooler or the like is increased. U-turn type layered hot fluid passages having corrugated fins and U-turn type layered cold fluid passages having corrugated fins are alternately stacked to configure a stacked core. Mixing headers are provided to both side surfaces of the stacked core so as to pass a hot fluid which circulates on a first half side and a return path side through the mixing headers outside of the stacked core. The mixing headers mix the hot fluid which circulates in the inside portion of the U-turn passage and the hot fluid which circulates in the outside portion thereof. The hot fluids are mixed between the plural stacked hot fluid passages. The temperature distribution of the hot fluid in the first half and in the passage width direction is made uniform.