A forked part structure of a double pipe has an outer pipe, an inner pipe, a branch pipe and an open end portion of the outer pipe. The open end portion of the outer pipe has a substantially tubular inner pipe supporting portion to which the inner pipe is brazed, and a substantially tubular branch pipe supporting portion to which the branch pipe is brazed. The inner pipe supporting portion is located inward of a main portion of the outer pipe along an extending direction of the main portion of the outer pipe.