The present invention provides a technique for alleviating not only a burden on a catheter but also a burden on an MDU part and for yet enabling the long-term use of the MDU part. For this purpose, a connector for connecting the catheter to the MDU part has therein a rotatable knob which exposes and holds an end surface of an optical fiber, an elastic body which exhibits a buffering function when this knob retreats due to an operation for connecting the catheter to the MDU part, and a member which is interposed between the knob and the elastic body, which is attached to the knob when the knob retreats, and which has a coefficient of friction equal to or lower than a predetermined value.
