A heat exchanger comprising: a plurality of heat exchange units (11, 11a, 11b, 11c, 11d, 11e, 11g, 11h, 11i) each including an inner pipe in which a first fluid flows, an insertion body inserted in the inner pipe, and at least one or more outer pipes in which a second fluid flows, the one or more outer pipes being provided on an outer periphery of the inner pipe; and a joint unit installed on the inner pipe, ends of the plurality of heat exchange units (11, 11a, 11b, 11c, 11d, 11e, 11g, 11h, 11i) being connected by the joint unit, wherein the insertion body includes a shaft and a helical protrusion placed on an outer surface of the shaft, the first fluid flows in a helical flow channel including an inner surface of the inner pipe, the shaft, and the helical protrusion, and at least two or more heat exchange units (11, 11a, 11b, 11c, 11d, 11 e, 11g, 11h, 11i) among the plurality of heat exchange units (11, 11a, 11b, 11c, 11d, 11 e, 11g, 11h, 11 i) are communicated by the joint unit, characterized in that the first fluid is branched by the joint unit and flows in the inner pipe in the same direction in at least two or more heat exchange units (11, 11a, 11b, 11c, 11d, 11e, 11g, 11h, 11i), wherein each of the plurality of heat exchange units (11, 11a, 11b, 11c, 11d, 11e, 11g, 11h, 11 i) includes a drain port that drains water inside each of the plurality of heat exchange units (11, 11a, 11b, 11c, 11d, 11e, 11g, 11h, 11i), wherein the joint unit includes a plurality of joints and a communication passage that makes the plurality of joints be communicated, and wherein the drain port is placed on any of the plurality of joints or the communication passage. a plurality of heat exchange units (11, 11a, 11b, 11c, 11d, 11e, 11g, 11h, 11i) each including an inner pipe in which a first fluid flows, an insertion body inserted in the inner pipe, and at least one or more outer pipes in which a second fluid flows, the one or more outer pipes being provided on an outer periphery of the inner pipe; and a joint unit installed on the inner pipe, ends of the plurality of heat exchange units (11, 11a, 11b, 11c, 11d, 11e, 11g, 11h, 11i) being connected by the joint unit, wherein the insertion body includes a shaft and a helical protrusion placed on an outer surface of the shaft, the first fluid flows in a helical flow channel including an inner surface of the inner pipe, the shaft, and the helical protrusion, and at least two or more heat exchange units (11, 11a, 11b, 11c, 11d, 11 e, 11g, 11h, 11i) among the plurality of heat exchange units (11, 11a, 11b, 11c, 11d, 11 e, 11g, 11h, 11 i) are communicated by the joint unit, the first fluid is branched by the joint unit and flows in the inner pipe in the same direction in at least two or more heat exchange units (11, 11a, 11b, 11c, 11d, 11e, 11g, 11h, 11i), wherein each of the plurality of heat exchange units (11, 11a, 11b, 11c, 11d, 11e, 11g, 11h, 11 i) includes a drain port that drains water inside each of the plurality of heat exchange units (11, 11a, 11b, 11c, 11d, 11e, 11g, 11h, 11i), wherein the joint unit includes a plurality of joints and a communication passage that makes the plurality of joints be communicated, and wherein the drain port is placed on any of the plurality of joints or the communication passage.