Abstract: A catheter a part which is inserted into and indwelled in a blood vessel by the Seldinger method and a sheath for covering the catheter are included. The catheter is constituted of a double lumen portion in which a first lumen and a second lumen are integrally formed in parallel with each other, a first branch tube and a second branch tube extended to the one side of the double lumen portion and an extension tube connecting portion provided to the other side of the double lumen portion. The inner cavity of the first branch tube communicates with the first lumen and the inner cavity of the second branch tube communicates with the second lumen. The distal end of the first branch tube is bulged in the vicinity thereof so as to slidably contact with the inside of the distal end of the sheath.