Abstract: In a medical catheter and a catheter assemble, a front opening 6a of a second lumen 6 is provided on a catheter 1. The front opening 6a acts as a lateral slope opening 8 inclined upward from a distal end T to a proximal end S along a common wall 7 of a dual lumen (5, 6). A second guide wire 10 extends its distal end 10a beyond the lateral slope opening 8. By operating the distal end 10a along a peripheral edge 8A of the lateral slope opening 8, it is possible to project a sphere in a three-dimensional perspective in which the distal end 10a can operationally move freely so as to extend a detectable region for a perforative hole against minute blood stream paths (n) developed in an occluded area of the blood vessel N.