A thin tube bundle (2) including a plurality of heat transfer thin tubes (1) is sealed by seal members (3a-3c) to form a blood channel (5) that crosses the heat transfer thin tubes. Heat transfer thin tube headers (6, 7) having an inlet port and an outlet port (6a, 7a) of heat medium liquid form flow chambers that contain ends of the thin tube bundle. The thin tube bundle is divided in a direction of the blood channel and forms a stack structure of thin tube bundle units (12a-12c). The flow chambers are partitioned into a plurality of flow compartments (13a, 13b, 14a, 14b) by partition walls (6b, 7b) to form a channel that allows heat medium liquid to pass through the respective thin tube bundle units successively via the flow compartments. An end of one of the thin tube bundle units on both sides of a border corresponding to the partition wall protrudes further than an end of the other thin tube bundle unit, and a side face of the partition wall contacts an side face of the protruding portion. Thus, the flow velocity of the heat medium liquid flowing through the heat transfer thin tubes is increased, and hence, heat exchange efficiency is enhanced while suppressing the increase in volume of the blood channel.
