A plate heat exchanger, comprising: a plurality of heat transfer plates each having a flat heat transfer surface; and a plurality of inner fins each being arranged in corresponding one of first channels or corresponding one of second channels, the first channels and the second channels being alternately formed between pairs of the plurality of heat transfer plates, respectively, wherein one of the plurality of inner fins, which is arranged in the corresponding one of the first channels, and another of the plurality of inner fins, which is arranged in the corresponding one of the second channels, are provided so as to have different heat transfer areas, leakage preventing plates are arranged between the pairs of the plurality of heat transfer plates, respectively, a passage hole through which at least a first fluid or a second fluid flows, is formed on each of the leakage preventing plates, wherein each of the leakage preventing plates has a flat portion in a region mated with the flat heat transfer surface of the corresponding one of the plurality of heat transfer plates, wherein each of the plurality of heat transfer plates and each of the leakage preventing plates are brought into surface contact with each other so as to hold each other, and wherein the each of the plurality of inner fins is structured such that projections and depressions are formed repeatedly, and wherein the projections and the depressions have flat end portions, characterized in that each of the plurality of inner fins is brought into surface contact with the corresponding heat transfer plate and leakage preventing plate at the flat end portions. a plurality of heat transfer plates each having a flat heat transfer surface; and a plurality of inner fins each being arranged in corresponding one of first channels or corresponding one of second channels, the first channels and the second channels being alternately formed between pairs of the plurality of heat transfer plates, respectively, wherein one of the plurality of inner fins, which is arranged in the corresponding one of the first channels, and another of the plurality of inner fins, which is arranged in the corresponding one of the second channels, are provided so as to have different heat transfer areas, leakage preventing plates are arranged between the pairs of the plurality of heat transfer plates, respectively, a passage hole through which at least a first fluid or a second fluid flows, is formed on each of the leakage preventing plates, wherein each of the leakage preventing plates has a flat portion in a region mated with the flat heat transfer surface of the corresponding one of the plurality of heat transfer plates, wherein each of the plurality of heat transfer plates and each of the leakage preventing plates are brought into surface contact with each other so as to hold each other, and wherein the each of the plurality of inner fins is structured such that projections and depressions are formed repeatedly, and wherein the projections and the depressions have flat end portions, characterized in that each of the plurality of inner fins is brought into surface contact with the corresponding heat transfer plate and leakage preventing plate at the flat end portions.