A plate heat exchanger comprising: a plurality of heat transfer plates in each of which a first through-hole, a second through-hole, a third through-hole, and a fourth through-hole are provided to extend through the heat transfer plate in a single direction, the plurality of heat transfer plates being stacked together in the single direction, configured to isolate a first flow passage through which a first fluid flows and a second flow passage through which a second fluid flows, and configured to cause heat exchange to be performed between the first fluid in the first flow passage and the second fluid in the second flow passage; a pair of end plates (43A, 43B) in which a first inlet port, a first outlet port, a second inlet port, a second outlet port, and a connection port are provided, the pair of end plates (43A, 43B) being provided such that the plurality of heat transfer plates are sandwiched between the pair of end plates (43A, 43B) in the single direction, the first inlet port being continuous with the first through-holes and located as a port through which the first fluid flows into the first flow passage, the first outlet port being continuous with the second through-holes and located as a port through which the first fluid flows out from the first flow passage, the second inlet port being continuous with the third through-holes and located as a port through which the second fluid flows into the second flow passage, the second outlet port being continuous with the fourth through-holes and located as a port through which the second fluid flows out from the second flow passage, the connection port being continuous with the first through-holes or the second through-holes, and branching off from the first flow passage, wherein the connection port is connected to a pressure relief valve provided separately from the plate heat exchanger, characterized in that the plate heat exchanger further comprises a temperature sensor configured to detect a temperature related to the first fluid that flows in the plate heat exchanger, wherein the temperature sensor is attached to one of the end plates (43A, 43B) that includes the connection port. a plurality of heat transfer plates in each of which a first through-hole, a second through-hole, a third through-hole, and a fourth through-hole are provided to extend through the heat transfer plate in a single direction, the plurality of heat transfer plates being stacked together in the single direction, configured to isolate a first flow passage through which a first fluid flows and a second flow passage through which a second fluid flows, and configured to cause heat exchange to be performed between the first fluid in the first flow passage and the second fluid in the second flow passage; a pair of end plates (43A, 43B) in which a first inlet port, a first outlet port, a second inlet port, a second outlet port, and a connection port are provided, the pair of end plates (43A, 43B) being provided such that the plurality of heat transfer plates are sandwiched between the pair of end plates (43A, 43B) in the single direction, the first inlet port being continuous with the first through-holes and located as a port through which the first fluid flows into the first flow passage, the first outlet port being continuous with the second through-holes and located as a port through which the first fluid flows out from the first flow passage, the second inlet port being continuous with the third through-holes and located as a port through which the second fluid flows into the second flow passage, the second outlet port being continuous with the fourth through-holes and located as a port through which the second fluid flows out from the second flow passage, the connection port being continuous with the first through-holes or the second through-holes, and branching off from the first flow passage, wherein the connection port is connected to a pressure relief valve provided separately from the plate heat exchanger, characterized in that the plate heat exchanger further comprises a temperature sensor configured to detect a temperature related to the first fluid that flows in the plate heat exchanger, wherein the temperature sensor is attached to one of the end plates (43A, 43B) that includes the connection port.