A gastrostomy tube extension device used for inserting or taking out the gastrostomy tube into/from the hole on the patient includes a rod, a fixing member, and an engaging member. The rod is formed of a rod-shaped member, and may be formed with engaging stepped portions on the proximal portion thereof. The fixing member is adapted to be 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, allowing the operator to manipulate the device with a single hand.