A gastrostomy tube extension device used for inserting or taking out a gastrostomy tube into/from a hole on a patient formed between the skin surface and the inner surface of a stomach wall; the gastrostomy tube comprising: an outer fixing member to be installed at the hole of the patient on the skin surface side, an inner fixing member to be installed on the inner side of the stomach wall which is formed of resilient material which becomes elongated and narrowed when the center of the distal end thereof is pushed toward the distal end; and a tube member for connecting the outer fixing member and the proximal portion of the inner fixing member, and a fluid substance feed hole extending from the outer fixing portion to the inner fixing portion, said extension device comprising: a pushing rod comprising a rod member (21 a) which can be inserted into the fluid substance feed hole, is capable of pushing the center portion of the distal end of the inner fixing member toward the distal end with the distal end portion thereof, and is formed with a positioning engaging portion on the peripheral surface of the proximal portion; and an engaging member including: a first engaging portion which is capable of engaging the outer fixing member, and a second engaging portion which is capable of engaging the positioning engaging portion of the pushing rod, the engaging member being arranged to elongate and narrow the inner fixing portion when the pushing rod is inserted into the fluid substance feed hole and the outer fixing member and the first engaging portion, and the positioning engaging portion and the second engaging portion are respectively engaged with each other, characterized in that a plurality of the positioning engaging portions are provided along the axial direction of the pushing rod and wherein each of the respective plurality of positioning engaging portions comprise an engaging step portion that is a ring-shaped projection. an outer fixing member to be installed at the hole of the patient on the skin surface side, an inner fixing member to be installed on the inner side of the stomach wall which is formed of resilient material which becomes elongated and narrowed when the center of the distal end thereof is pushed toward the distal end; and a tube member for connecting the outer fixing member and the proximal portion of the inner fixing member, and a fluid substance feed hole extending from the outer fixing portion to the inner fixing portion, a pushing rod comprising a rod member (21 a) which can be inserted into the fluid substance feed hole, is capable of pushing the center portion of the distal end of the inner fixing member toward the distal end with the distal end portion thereof, and is formed with a positioning engaging portion on the peripheral surface of the proximal portion; and an engaging member including: a first engaging portion which is capable of engaging the outer fixing member, and a second engaging portion which is capable of engaging the positioning engaging portion of the pushing rod, the engaging member being arranged to elongate and narrow the inner fixing portion when the pushing rod is inserted into the fluid substance feed hole and the outer fixing member and the first engaging portion, and the positioning engaging portion and the second engaging portion are respectively engaged with each other, characterized in that a plurality of the positioning engaging portions are provided along the axial direction of the pushing rod and wherein each of the respective plurality of positioning engaging portions comprise an engaging step portion that is a ring-shaped projection.