A plate fin heat exchanger, comprising: a heat exchange unit inside which a plurality of flow passages is provided so as to be layered and heat exchange is performed between fluids flowing through the flow passages; and a pair of protection units arranged on both outer sides of said heat exchange unit in the layering direction of the flow passages, the protection units for protecting said heat exchange unit, wherein said heat exchange unit has a plurality of partition walls aligned at intervals in such a manner that the flow passage is formed between said adjacent partition walls, and a heat exchange unit fin plate which is arranged in the flow passage for coupling said partition walls facing each other across the flow passage, and said protection unit has a protection unit main body attached to the outermost partition wall so as to form an internal space between said protection unit main body and said outermost partition wall, and a protection unit fin plate arranged in the internal space of said protection unit main body, the protection unit fin plate for coupling an outer surface of said outermost partition wall and an inner surface of said protection unit main body facing the outer surface, wherein said protection unit fin plate of said protection unit has such strength that a coupling state between the outer surface of said outermost partition wall and the inner surface of said protection unit main body facing the outer surface is maintained even in a case where an inner pressure set as a design pressure for a part of said heat exchange unit constituting the outermost-layer flow passage adjacent to said protection unit is applied to the internal space of said protection unit main body of said protection unit, characterized in that a release portion for, in a case where the fluid flows into the internal space, releasing the fluid to an exterior is provided in said protection unit main body of said protection unit, wherein the release portion is adapted such that, in a case where a damage such as breaking or cracking is generated in the outermost partition wall and fluid flows into the internal space of the protection unit main body, this fluid flowing into the internal space of the protection unit main body is emitted to the atmosphere through the release portion. a heat exchange unit inside which a plurality of flow passages is provided so as to be layered and heat exchange is performed between fluids flowing through the flow passages; and a pair of protection units arranged on both outer sides of said heat exchange unit in the layering direction of the flow passages, the protection units for protecting said heat exchange unit, wherein said heat exchange unit has a plurality of partition walls aligned at intervals in such a manner that the flow passage is formed between said adjacent partition walls, and a heat exchange unit fin plate which is arranged in the flow passage for coupling said partition walls facing each other across the flow passage, and said protection unit has a protection unit main body attached to the outermost partition wall so as to form an internal space between said protection unit main body and said outermost partition wall, and a protection unit fin plate arranged in the internal space of said protection unit main body, the protection unit fin plate for coupling an outer surface of said outermost partition wall and an inner surface of said protection unit main body facing the outer surface, said protection unit fin plate of said protection unit has such strength that a coupling state between the outer surface of said outermost partition wall and the inner surface of said protection unit main body facing the outer surface is maintained even in a case where an inner pressure set as a design pressure for a part of said heat exchange unit constituting the outermost-layer flow passage adjacent to said protection unit is applied to the internal space of said protection unit main body of said protection unit, characterized in that a release portion for, in a case where the fluid flows into the internal space, releasing the fluid to an exterior is provided in said protection unit main body of said protection unit, wherein the release portion is adapted such that, in a case where a damage such as breaking or cracking is generated in the outermost partition wall and fluid flows into the internal space of the protection unit main body, this fluid flowing into the internal space of the protection unit main body is emitted to the atmosphere through the release portion.