A medical instrument guiding device (outer tube) to be tapped into a body wall includes: an endoscope insertion hole for inserting the endoscope； a treatment tool insertion hole for inserting the treatment tool； and an interlocking mechanism for moving the endoscope back and forth in interlock with the back-and-forth movement of the treatment tool. The interlocking mechanism includes an endoscope-side roller contacts with an endoscope insertion part and moves in interlock with the endoscope insertion part, and a treatment tool-side roller which contacts with a treatment tool insertion part and moves in interlock with the treatment tool insertion part, and those rollers rotates in interlock with each other. In response to the movement of the treatment tool insertion part, the treatment tool-side roller and the endoscope-side roller rotate in an opposite direction to move the endoscope insertion part.
