The present subject matter includes: a heat exchange part having heating medium channels, through which heating medium flows, and combustion gas channels, through which combustion gas burned in a burner flows, adjacently disposed in alternation in the spaces between the plurality of plates; a first welding part in which the outer edges of a pair of plates forming the heating medium channel are welded together; a second welding part in which the plates forming the combustion gas channel, between the plates forming the heating medium channel, are welded to the outer edges of the plates forming the heating medium channel to form; and a guide part for guiding the welding material molten solution of the first welding part into moving to the second welding part.