To treat particularly large quantity of a sea water or chemicals by guiding a first heat exchanging medium to one of an inner surface side and an outer surface side of an element, guiding a second heat exchanging medium to the other, and heat exchanging the one medium with the other.

This plate type heat exchanger comprises many elements 2 arranged apart at an interval. In this embodiment, a sea water flows as a liquid 5 from below to above in each element 2. The each element 2 has a pair of plates 1 oppositely disposed near at hand, and its four peripheral sides, are blocked by a sealing member. As the plate 1, a wavy plate of a polyvinyl chloride can be used. A waveform polyvinyl chloride plate is connected between the pair of flat surface-like plates as the plate 1. Further, the element 1 may be formed in a honeycomb structure.