A plate heat exchanger includes: a plurality of heat transfer plates in each of which first to fourth through-holes are provided to extend through the heat transfer plate in a single direction, and which are stacked together in the single direction and provided to isolate first and second water flow passages; and a pair of end plates in which first and second inlet ports, first and second outlet ports, and a connection port are provided, and which are provided such that the heat transfer plates are sandwiched between the end plates in the single direction. The first inlet port is continuous with the first through-holes and allows a first fluid to flow into the first flow passage. The first outlet port is continuous with the second through-holes and allow the first fluid to flow out from the first flow passage. The second inlet port is continuous with the third through-holes and allows a second fluid to flow into the second flow passage. The second outlet port is continuous with the fourth through-holes and allows the second fluid to flow out from the second flow passage. The connection port is connected to a pressure relief valve provided separately from the plate heat exchanger.