A catheter comprising a sheath to be inserted into a living body, wherein the sheath includes a tubular reinforcement layer of at least one layer, which is formed with a spiral slit continuous from the distal side to the proximal side thereof; a termination end of the spiral slit is provided on the proximal side of the slit proximal portion at the site of the proximal side of the spiral slit; and at the same time, there is formed a slit termination portion in which inclination angle of the spiral slit with respect to the circumferential direction of the reinforcement layer is larger than that of the slit proximal portion.