A heat exchanger comprising: a first pipe including a first flow path allowing a first heat medium to flow; and a second pipe including a second flow path allowing a second heat medium to flow, the second pipe being wound around the first pipe, wherein the first pipe includes: at least one ridge (3a, 3a1, 3a2, 3a3, 3a4) protruding in a diameter-increasing direction in which a diameter of the first pipe increases; and at least one trough (3b, 3b1, 3b2, 3b3, 3b4) having an outer diameter smaller than an outer diameter of a portion of the first pipe where the ridge (3a, 3a1, 3a2, 3a3, 3a4) is formed, the trough (3b, 3b1, 3b2, 3b3, 3b4) allowing the second pipe to be wound thereon, the ridge (3a, 3a1, 3a2, 3a3, 3a4) is formed helically in a flow direction of the first heat medium in the first flow path, the trough (3b, 3b1, 3b2, 3b3, 3b4) is formed helically along the ridge (3a, 3a1, 3a2, 3a3, 3a4), characterized in that the trough (3b, 3b1, 3b2, 3b3, 3b4) includes a recess along a helical direction in which the trough (3b, 3b1, 3b2, 3b3, 3b4) runs, the recess being depressed in a diameter-decreasing direction in which the diameter of the first pipe decreases, and when an n-th turn of a helix of the ridge (3a, 3a1, 3a2, 3a3, 3a4) is defined as a first ridge (3a-1), an (n+1)-th turn of the helix of the ridge (3a, 3a1, 3a2, 3a3, 3a4) is defined as a second ridge (3a-2), and the trough (3b, 3b1, 3b2, 3b3, 3b4) between the first ridge (3a-1) and the second ridge (3a-2) is defined as an intermediate trough, and when a portion of the first pipe including the first ridge (3a-1), the second ridge (3a-2), and the intermediate trough is viewed in a section taken along the flow direction of the first heat medium, the recess is formed such that a vertex of the recess is positioned downstream of a center point of the intermediate trough in the flow direction of the first heat medium. a first pipe including a first flow path allowing a first heat medium to flow; and a second pipe including a second flow path allowing a second heat medium to flow, the second pipe being wound around the first pipe, wherein the first pipe includes: at least one ridge (3a, 3a1, 3a2, 3a3, 3a4) protruding in a diameter-increasing direction in which a diameter of the first pipe increases; and at least one trough (3b, 3b1, 3b2, 3b3, 3b4) having an outer diameter smaller than an outer diameter of a portion of the first pipe where the ridge (3a, 3a1, 3a2, 3a3, 3a4) is formed, the trough (3b, 3b1, 3b2, 3b3, 3b4) allowing the second pipe to be wound thereon, at least one ridge (3a, 3a1, 3a2, 3a3, 3a4) protruding in a diameter-increasing direction in which a diameter of the first pipe increases; and at least one trough (3b, 3b1, 3b2, 3b3, 3b4) having an outer diameter smaller than an outer diameter of a portion of the first pipe where the ridge (3a, 3a1, 3a2, 3a3, 3a4) is formed, the trough (3b, 3b1, 3b2, 3b3, 3b4) allowing the second pipe to be wound thereon, the ridge (3a, 3a1, 3a2, 3a3, 3a4) is formed helically in a flow direction of the first heat medium in the first flow path, the trough (3b, 3b1, 3b2, 3b3, 3b4) is formed helically along the ridge (3a, 3a1, 3a2, 3a3, 3a4), characterized in that the trough (3b, 3b1, 3b2, 3b3, 3b4) includes a recess along a helical direction in which the trough (3b, 3b1, 3b2, 3b3, 3b4) runs, the recess being depressed in a diameter-decreasing direction in which the diameter of the first pipe decreases, and when an n-th turn of a helix of the ridge (3a, 3a1, 3a2, 3a3, 3a4) is defined as a first ridge (3a-1), an (n+1)-th turn of the helix of the ridge (3a, 3a1, 3a2, 3a3, 3a4) is defined as a second ridge (3a-2), and the trough (3b, 3b1, 3b2, 3b3, 3b4) between the first ridge (3a-1) and the second ridge (3a-2) is defined as an intermediate trough, and when a portion of the first pipe including the first ridge (3a-1), the second ridge (3a-2), and the intermediate trough is viewed in a section taken along the flow direction of the first heat medium, the recess is formed such that a vertex of the recess is positioned downstream of a center point of the intermediate trough in the flow direction of the first heat medium.