PURPOSE: To obtain the catheter which can be used for middle and large intravascular cavities by forming at least one or more rings in one opening part of a salpinx, and attaching one or more operating wires to at least one or more rings.

CONSTITUTION: At the time of using this catheter, the tip part of the catheter is deformed so as to become flat, and so that it can easily be inserted into a sheath jacket by inserting a sheath set jacket into a target intravascular cavity, and for instance, by executing a drawing operation of the end of an operation wire 2A. After it is inserted into an inner cavity of the sheath jacket in this deformed state, subsequently, the operating wires 2A, 2B are pushed forward so that they are pushed through in the sheath jacket, and when a ring 1 of the tip is extruded from the sheath jacket, the operating wire 2A is returned to its original position, and the ring 1 is reset to its original shape, and fixed in an expanded state in the intravascular cavity. Thereafter, a medical treatment of a lesion is executed by inserting an intravascular cavity surgery implement into a salpinx 3.