A transendoscopic medical instrument comprising: a sheath configured to be inserted into and removed from a channel of an endoscope and includes a distal end opening portion; an x-ray marker member formed of a radiopaque material being able to be arranged in the sheath; a guide unit configured to be inserted into and removed from the sheath, and has a distal end portion configured to push the x-ray marker member when inserted in the sheath and is retractable from the distal end opening portion of the sheath; and a latching means provided to the sheath and the guide unit configured to latch the guide unit with respect to the sheath, the x-ray marker member is configured to be discharged from the distal end opening portion of the sheath through the distal end opening portion when a predetermined pushing force is applied, wherein a tapered portion is adapted to elastically expand when applying the pushing force; characterized in that: the x-ray marker member is latched with respect to the distal end opening at the time of no load by the tapered portion of the sheath; wherein at least an inside diameter at the distal end portion in the distal end opening portion is tapered to form the tapered portion of the sheath, with the x-ray marker member configured to be discharged from the distal end opening portion of the sheath through the tapered portion of the distal end opening portion, wherein the tapered portion is adapted to elastically expand when applying the pushing force; the guide unit comprises a flexure portion which is operated by a flexural operating means and can change a direction of the distal end portion; and the latching means has a plurality of latching portions with each latching portion configured to hold the distal end portion of the guide unit with respect to the sheath in one of a first position where the x-ray marker member and the guide unit are inserted into the sheath, a second position where the x-ray marker member is completely discharged from the sheath and a third position where the flexure portion of the guide unit completely protrudes from the distal end opening portion of the sheath. a sheath configured to be inserted into and removed from a channel of an endoscope and includes a distal end opening portion; an x-ray marker member formed of a radiopaque material being able to be arranged in the sheath; a guide unit configured to be inserted into and removed from the sheath, and has a distal end portion configured to push the x-ray marker member when inserted in the sheath and is retractable from the distal end opening portion of the sheath; and a latching means provided to the sheath and the guide unit configured to latch the guide unit with respect to the sheath, the x-ray marker member is configured to be discharged from the distal end opening portion of the sheath through the distal end opening portion when a predetermined pushing force is applied, wherein a tapered portion is adapted to elastically expand when applying the pushing force; characterized in that: the x-ray marker member is latched with respect to the distal end opening at the time of no load by the tapered portion of the sheath; wherein at least an inside diameter at the distal end portion in the distal end opening portion is tapered to form the tapered portion of the sheath, with the x-ray marker member configured to be discharged from the distal end opening portion of the sheath through the tapered portion of the distal end opening portion, wherein the tapered portion is adapted to elastically expand when applying the pushing force; the guide unit comprises a flexure portion which is operated by a flexural operating means and can change a direction of the distal end portion; and the latching means has a plurality of latching portions with each latching portion configured to hold the distal end portion of the guide unit with respect to the sheath in one of a first position where the x-ray marker member and the guide unit are inserted into the sheath, a second position where the x-ray marker member is completely discharged from the sheath and a third position where the flexure portion of the guide unit completely protrudes from the distal end opening portion of the sheath. the x-ray marker member is latched with respect to the distal end opening at the time of no load by the tapered portion of the sheath; wherein at least an inside diameter at the distal end portion in the distal end opening portion is tapered to form the tapered portion of the sheath, with the x-ray marker member configured to be discharged from the distal end opening portion of the sheath through the tapered portion of the distal end opening portion, wherein the tapered portion is adapted to elastically expand when applying the pushing force; the guide unit comprises a flexure portion which is operated by a flexural operating means and can change a direction of the distal end portion; and the latching means has a plurality of latching portions with each latching portion configured to hold the distal end portion of the guide unit with respect to the sheath in one of a first position where the x-ray marker member and the guide unit are inserted into the sheath, a second position where the x-ray marker member is completely discharged from the sheath and a third position where the flexure portion of the guide unit completely protrudes from the distal end opening portion of the sheath.