Locking device that is mounted on an endoscope or the like for selectively securing the position of a guide wire and/or catheter relative to the endoscope or the like. The locking device preferably includes a side wall with an opening therein for receiving the proximal end of a guide wire or catheter. The opening is preferably J-shaped or boot shaped, and has an entry end and a locking end. Once a guide wire or catheter is in a desired position within a body cavity, the portion of the guide wire or catheter that extends outside of the endoscope or the like may be moved into the opening. More particularly, a portion of the guide wire or catheter may be inserted by an operator through the entry end of the opening and into the locking end, wherein the locking end frictionally secures the position of the guide wire or catheter relative to the endoscope or the like.