Provided is a catheter assembly that can increase the degree of freedom for selecting a guide wire to be used. The catheter assembly includes an outer catheter and an inner catheter. The outer catheter is a catheter for introducing a balloon catheter or the like to an affected area and the inner catheter is an insertion-assisting tool that assists insertion by preceding the insertion of the outer catheter when being inserted in a human body. An inner opening by which a guide wire tube hole of the inner catheter is opened at a proximal end side and an outer opening by which the outer tube hole of the outer catheter is opened at a proximal end side are formed at the middle position in the axial line direction, and the openings are formed at an angle with respect to the axial line directions at positions on the same line.
