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
