A plate heat exchanger includes a plurality of rectangular plates each having, at four corners thereof, inlets and outlets and others for a first fluid and a second fluid. The plates are stacked such that first passages each defined by adjacent two of the plates and through which the first fluid flows and second passages each defined by adjacent two of the plates and through which the second fluid flows are provided alternately. The first passage includes a bypass passage extending from an inlet peripheral portion, which is an area around the inlet, along the outlet for the second fluid up to a long-side-peripheral portion of the plate that is nearer to the second outlet. The bypass passage allows some of the first fluid having flowed therein from the inlet to flow from the long-side-peripheral portion into a heat-exchanging passage.