A heat exchanger for a ventilation system which includes an air exhaust duct and an air supply duct positioned at both ends of the heat exchanger so as to discharge contaminated inside air to the outside and to introduce fresh outside air to the inside and which introduces outside air by cooling/heating the outside air by using cool/warm energy being likely to be lost when discharging the inside air, the heat exchanger for performing heat exchange between the exhaust duct and the air supply duct, comprising a first heat transfer plate; and a second heat transfer plate, and wherein a number of the first heat transfer plates and a number of the second heat transfer plates are stacked alternately so as to be correspondingly combined with each other, wherein the first heat transfer plate, which includes a front side, a back side, two left sides respectively having a first sloping end and a second sloping end, and two right sides respectively having a first sloping end and a second sloping end, wherein a first closing groove formed between the horizontal edge protruding upward from a base level and a projection spaced apart from the horizontal edge, at a predetermined distance, and protruding upward from the horizontal edge at each of the first sloping ends of the left and right sides; and a second closing projection spaced apart from the horizontal edge, at a predetermined distance, at each of the second sloping ends of the left and right sides and hollowed out downward from the base line; and wherein the second heat transfer plate which includes a front side, a back side, two left sides respectively having a first sloping end and a second sloping end, and two right sides respectively having a first sloping end and a second sloping end and which is correspondingly combined with the first heat transfer plate on and/or under the first heat transfer plate, and wherein a first closing projection is formed at each of the first sloping ends of the left and right sides of the second heat transfer plate and fitted into the first closing groove, to block a fluid flow passage of the cool air/hot air; and a second closing groove is formed at each of the second sloping ends of the left and right sides of the second heat transfer plate and correspondingly combined with the second closing projection, and wherein the first heat transfer plate and the second heat transfer plate forms an inlet/outlet for introducing/discharging the cold/hot air, at the sloping ends of the left and right sides where each second closing projection is formed, characterized in that the first heat transfer plate comprises a first combining projection formed at each of edges of the front and back sides, the first combining projection spaced apart from a horizontal edge, at a predetermined distance, and protruding upward from the base level so as to be relatively higher than a projection, and a second combining projection formed inward the first combining projection and having a shape hollowed out downward from the base level, and that the second heat transfer plate comprises a first combining groove formed at each of edges of the front and back sides of the second heat transfer plate and receiving each first combining projection so as to be combined with each other, and a second combining groove formed at each of the edges of the front and back sides of the second heat transfer plate and corresponding to the second combining projection.