A gastrostomy tube extension device which can facilitate the insertion or removal operation in and from the patient, and increase the simplicity of manufacture. A gastrostomy tube extension device for inserting or taking out the gastrostomy tube into/from the patient includes a rod, a fixing member, and an engaging member. The rod is formed of a rod-shaped member with engaging stepped portions. The fixing member is capable of moving along the longitudinal direction of the rod, and an engaging projection which can be fixed to the tube member is formed on the peripheral surface of the fixing member. The engaging member includes a lower engaging portion engageable with the engaging projection and an upper engaging portion which is engageable with the engaging stepped portion This assembly allows the extension to be locked.