A plate heat exchanger comprising: first and second plates including first plates each of which has a rectangular shape and second plates each of which has a rectangular shape, the first plates and the second plates being stacked alternately on one another and each being provided with passage holes at four corners thereof, the passage holes serving as inlets and outlets for a first fluid and a second fluid; and first and second passages including first passages each of which is a passage through which the first fluid flows and second passages each of which is a passage through which the second fluid flows, the first passages and the second passages being formed alternately one another between adjacent ones of the first plates and corresponding ones of the second plates in a stacking direction, wherein each of the first passages allows the first fluid having flowed therein from a first inlet to flow out of a first outlet, the first inlet being one of the passage holes that is on a first side in a long-side direction of each of the first plates and each of the second plates, the first outlet being one of the passage holes that is on a second side in the long-side direction, the first passage including a heat-exchanging passage that extends between the first inlet and the first outlet and allows the first fluid to exchange heat with the second fluid flowing in a corresponding one of the second passages adjacent to the first passage, wherein each of the first plates includes, in the heat-exchanging passage thereof, first waves formed in a corrugated portion, protruding in the stacking direction and including a plurality of top parts and a plurality of bottom parts alternately formed in a repeated manner from the first inlet toward the first outlet; and second waves formed in a corrugated portion, protruding in the stacking direction and connected to the first waves, the second waves being formed on a side of an upstream-side adjacent hole in the heat-exchanging passage, the upstream-side adjacent hole being another one of the passage holes that is on the first side in the long-side direction and is different from the first inlet, characterized in that the top parts of the first waves and top parts of the second waves have respective planar shapes, and the top parts of the first waves have a larger top width than the top parts of the second waves, the top width being a width in a direction perpendicular to ridges of each corrugated portion. first and second plates including first plates each of which has a rectangular shape and second plates each of which has a rectangular shape, the first plates and the second plates being stacked alternately on one another and each being provided with passage holes at four corners thereof, the passage holes serving as inlets and outlets for a first fluid and a second fluid; and first and second passages including first passages each of which is a passage through which the first fluid flows and second passages each of which is a passage through which the second fluid flows, the first passages and the second passages being formed alternately one another between adjacent ones of the first plates and corresponding ones of the second plates in a stacking direction, wherein each of the first passages allows the first fluid having flowed therein from a first inlet to flow out of a first outlet, the first inlet being one of the passage holes that is on a first side in a long-side direction of each of the first plates and each of the second plates, the first outlet being one of the passage holes that is on a second side in the long-side direction, the first passage including a heat-exchanging passage that extends between the first inlet and the first outlet and allows the first fluid to exchange heat with the second fluid flowing in a corresponding one of the second passages adjacent to the first passage, wherein each of the first plates includes, in the heat-exchanging passage thereof, first waves formed in a corrugated portion, protruding in the stacking direction and including a plurality of top parts and a plurality of bottom parts alternately formed in a repeated manner from the first inlet toward the first outlet; and second waves formed in a corrugated portion, protruding in the stacking direction and connected to the first waves, the second waves being formed on a side of an upstream-side adjacent hole in the heat-exchanging passage, the upstream-side adjacent hole being another one of the passage holes that is on the first side in the long-side direction and is different from the first inlet, characterized in that the top parts of the first waves and top parts of the second waves have respective planar shapes, and the top parts of the first waves have a larger top width than the top parts of the second waves, the top width being a width in a direction perpendicular to ridges of each corrugated portion.