A plate heat exchanger comprising: a plurality of heat transfer plates (2, 3) respectively having through holes (202, 203) penetrating therethrough in a certain direction at positions corresponding to each other, the plurality of heat transfer plates being stacked on each other in the certain direction to alternately form first flow channels (Ra) through which a first fluid medium is circulated and second flow channels (Rb) through which a second fluid medium is circulated, with the plurality of heat transfer plates (2, 3) respectively interposed therebetween; and a flow channel forming member group extending in the certain direction at the position corresponding to the through holes (202, 203) of the plurality of heat transfer plates (2, 3), wherein the flow channel forming member group comprises a plurality of flow channel forming members lined up in the certain direction and arranged to be placed between circumferential portions of the through holes (202, 203) of each adjacent heat transfer plates (2, 3), at least two flow channel forming members out of the plurality of flow channel forming members respectively have through holes penetrating therethrough in the certain direction, the through holes of the at least two flow channel forming members communicate with each other to form a first fluid medium supply channel for supplying the first fluid medium to the first flow channels (Ra), and the first fluid medium supply channel comprises: an introduction part that extends in the certain direction and through which the first fluid medium is externally introduced; a first branching part that is arranged at an intermediate portion of the plurality of heat transfer plates (2, 3) aligned in the certain direction and that allows the first fluid medium introduced through the introduction part to branch to one side and an other side in the certain direction; and a plurality of opening parts directly or indirectly communicating with the one side or the other side of the first branching part, the plurality of opening parts each open toward a corresponding one of the first flow channels at a plurality of locations in the certain direction. a plurality of heat transfer plates (2, 3) respectively having through holes (202, 203) penetrating therethrough in a certain direction at positions corresponding to each other, the plurality of heat transfer plates being stacked on each other in the certain direction to alternately form first flow channels (Ra) through which a first fluid medium is circulated and second flow channels (Rb) through which a second fluid medium is circulated, with the plurality of heat transfer plates (2, 3) respectively interposed therebetween; and a flow channel forming member group extending in the certain direction at the position corresponding to the through holes (202, 203) of the plurality of heat transfer plates (2, 3), wherein the flow channel forming member group comprises a plurality of flow channel forming members lined up in the certain direction and arranged to be placed between circumferential portions of the through holes (202, 203) of each adjacent heat transfer plates (2, 3), at least two flow channel forming members out of the plurality of flow channel forming members respectively have through holes penetrating therethrough in the certain direction, the through holes of the at least two flow channel forming members communicate with each other to form a first fluid medium supply channel for supplying the first fluid medium to the first flow channels (Ra), and the first fluid medium supply channel comprises: an introduction part that extends in the certain direction and through which the first fluid medium is externally introduced; a first branching part that is arranged at an intermediate portion of the plurality of heat transfer plates (2, 3) aligned in the certain direction and that allows the first fluid medium introduced through the introduction part to branch to one side and an other side in the certain direction; and a plurality of opening parts directly or indirectly communicating with the one side or the other side of the first branching part, the plurality of opening parts each open toward a corresponding one of the first flow channels at a plurality of locations in the certain direction. an introduction part that extends in the certain direction and through which the first fluid medium is externally introduced; a first branching part that is arranged at an intermediate portion of the plurality of heat transfer plates (2, 3) aligned in the certain direction and that allows the first fluid medium introduced through the introduction part to branch to one side and an other side in the certain direction; and a plurality of opening parts directly or indirectly communicating with the one side or the other side of the first branching part, the plurality of opening parts each open toward a corresponding one of the first flow channels at a plurality of locations in the certain direction.