A plate fin heat exchanger of the present invention includes a heat exchange part including a heat exchange part main body including layers of plural flow passages, and heat transfer members each of which is disposed within each flow passage of the heat exchange part main body to transfer the heat of fluid flowing in each of the flow passages to each partition walls opposed across the flow passage; and sensing parts connected to both the outsides of the heat exchange part respectively. Each of the sensing parts includes plural sealed spaces, and a sensor wall disposed to separate the outermost sealed space from the sealed space on the inner side thereof. The plate fin heat exchanger further includes a detection means for detecting damage of the sensor wall of the sensing part. According to such a structure, external leak of the fluid performing the heat exchange can be prevented while suppressing deterioration of performance or increase in size or weight.