a heat exchanger comprising : a corrugated plate ; a base plate ; a cover plate ; a plurality of flow paths a and a plurality of flow pats b defined by the corrugated plate , the base plate and the cover plate disposed so as to interpose said corrugated plate therebetween ; and the plurality of flow paths a and the plurality of flow paths b being alternately and adjacently arranged on both sides of the corrugated plate for exchanging heat between fluid a and fluid b via the corrugated plate by supplying fluid a through a plurality of flow paths a and fluid b through a plurality of flow paths b , wherein at least one of end portions of the plurality of flow paths is hermetically sealed by making partial cuts at the end portions of the corrugated plate and / or folding a part of the end portions of the corrugated plate , bending opposing ends of the corrugated plate so as to form respectively into a se of a gable roof , bringing at least a part of the ends of the corrugated plate into contact with each other , and brazing portions in contact , so that openings of one of the flow paths a and the flow paths b are hermetically sealed for allowing only one of the fluids to flow in / out at each end of the corrugated plate .