A heat treatment device (1) utilizing heat exchange between a first fluid (M) and a second fluid (HC) includes first heat transfer bodies (7) including first flow channels (17) through which the first fluid (M, P) flows, second heat transfer bodies (9) including second flow channels (31) through which the second fluid (HC) flows, the second heat transfer bodies (9) each being stacked on the respective first heat transfer bodies (7), and a casing (53, 57) having a space (S2, S3) communicating with the second flow channels (31), the casing (53, 57) being in contact with each surface including the edge of the connection interface between each first heat transfer body (7) and each second heat transfer body (9). The first heat transfer bodies (7) each further include a third flow channel (18, 24) provided in a wall portion (19, 13) isolating the first flow channels (17) from the space (S2, S3) of the casing (53, 57). The first flow channels (17) are grooves in contact with the connection interface, and the third flow channel (18, 24) is a groove in contact with the connection interface and intersecting with a virtual line (LI, L2) connecting the first flow channels with the space (S2, S3) of the casing (53, 57) at the connection interface.