A double pipe heat exchanger comprising: an inner pipe inside which a first fluid passes; an outer pipe that has a diameter larger than a diameter of the inner pipe and that covers the inner pipe , the inner pipe and the outer pipe configured to allow a second fluid to pass through a space defined therebetween; and a heat transfer area enlargement pipe that is provided in the space and that has a projection-depression shape in which, in a pipe cross section, an angle between the heat transfer area enlargement pipe and the outer wall of the inner pipe at each of inner contact portions to be contact areas between an outer wall of the inner pipe and the heat transfer area enlargement pipe is smaller than an angle between the heat transfer area enlargement pipe and the inner wall of the outer pipe at each of outer contact portions to be contact areas between an inner wall of the outer pipe and the heat transfer area enlargement pipe , in which, in the pipe cross section, a length in a pipe-circumferential direction of the inner contact portions to be contact areas with the outer wall of the inner pipe is made to be larger than a length in a pipe-circumferential direction of outer contact portions to be contact areas with the inner wall of the outer pipe , and in which, fin portions between the inner contact portions and the outer contact portions that traverse the space in the pipe cross section come into contact with the outer wall of the inner pipe and the inner wall of the outer pipe in oblique directions, characterised in that the fin portions each have an arc shape in the pipe cross section. an inner pipe inside which a first fluid passes; an outer pipe that has a diameter larger than a diameter of the inner pipe and that covers the inner pipe , the inner pipe and the outer pipe configured to allow a second fluid to pass through a space defined therebetween; and a heat transfer area enlargement pipe that is provided in the space and that has a projection-depression shape in which, in a pipe cross section, an angle between the heat transfer area enlargement pipe and the outer wall of the inner pipe at each of inner contact portions to be contact areas between an outer wall of the inner pipe and the heat transfer area enlargement pipe is smaller than an angle between the heat transfer area enlargement pipe and the inner wall of the outer pipe at each of outer contact portions to be contact areas between an inner wall of the outer pipe and the heat transfer area enlargement pipe , in which, in the pipe cross section, a length in a pipe-circumferential direction of the inner contact portions to be contact areas with the outer wall of the inner pipe is made to be larger than a length in a pipe-circumferential direction of outer contact portions to be contact areas with the inner wall of the outer pipe , and in which, fin portions between the inner contact portions and the outer contact portions that traverse the space in the pipe cross section come into contact with the outer wall of the inner pipe and the inner wall of the outer pipe in oblique directions, characterised in that the fin portions each have an arc shape in the pipe cross section. <EOS>