To provide a welded plate type heat exchanger capable of exchanging heat of a fluid uniformly.

The welded plate type heat exchanger 30 is characterized by that plates 14, 14' are further laminated on both sides of a lamination direction of a plate cassette 12, and a second fluid distribution passage L2 for circulating a second fluid for exchanging heat is further formed on an outer side of the plate cassettes 12a, 12b on both ends. Thereby, a first fluid circulating in a first fluid distribution passage L1 in the plate cassettes 12a, 12b on both of the ends can exchange heat with the second fluid circulating in the second fluid distribution passage L2 on both sides as like the first fluid passing the first fluid distribution passage L1 in the other plate cassette 12. Thus, more uniform heat exchange can be performed.