In the field of separating substances using the flow of fluid, it is required to reduce the inside diameter of a separating pipe as the quantity of an organic solvent being used is reduced. Furthermore, the concentration of a target sample increases as the flow rate thereof decreases and thereby high sensitivity analysis is ensured. But a thin separating pipe is difficult to produce and susceptible to fracture. The inventive separation pipe has double pipe structure of inner pipe and outer pipe wherein the outer pipe can be split and fitted removably. Furthermore, a union is screwed to the end of the outer pipe in order to prevent strain and torsion between the inner and outer pipes and to eliminate dead volume.