A dual pipe according to present invention includes an outer pipe having a valley/ridge portion on an outer circumferential surface thereof, and an inner pipe having a valley/ridge portion formed on an inner circumferential surface thereof and inserted into the outer pipe. In the dual pipe, the inner pipe and the outer pipe are threadedly engaged with each other. A low pressure refrigerant passes through the inner pipe. In order to secure a passage of a high pressure refrigerant in a space between the outer circumferential surface of the inner pipe and the inner circumferential surface of the outer pipe, a part of the helical valley/ridge portion formed on the outer circumferential surface of the inner pipe is composed of a multiple-helix helical valley/ridge portion. A part of the helical valley/ridge portion formed on the inner circumferential surface of the outer pipe is composed of a single-helix helical valley/ridge portion (more precisely, a helical valley/ridge portion having a smaller number of helixes than the number of helixes of the multiple-helix helical valley/ridge portion of the inner pipe). That is, the high-pressure refrigerant flows through a space between a valley of a non-threadedly-engaged helical valley/ridge portion of the multiple-helix helical valley/ridge portion, which is formed on the outer circumferential surface of the inner pipe, and the outer pipe.