Grout bag reinforcing method for underground caves

A column construction method using a grout bag is provided to form a large diameter of column in an underground cavity by boring a small diameter hole on the upper ground, to construct a column having constant diameter at the upper and lower parts to support the upper ground of the underground cavity, and to maintain structural stability by prevent a grout loss caused by underground water flow in the underground cavity without usage of a grout mixer or additional reinforcing construction. A column construction method using a grout bag for supporting an upper ground(20) of an underground cavity comprises steps for boring a hole(10) on the upper ground from the ground surface to the ceiling unit of the underground cavity; preparing an injection pipe(100) having a discharge unit formed with plural discharge holes(110) on one end; surrounding the discharge unit of the injection pipe with a grout bag(200) and combining an entry of the grout bag with a sealing band(300) to mount the grout bag on the discharge unit of the injection pipe; inserting the injection pipe through the hole into the underground cavity; and expanding the grout bag and forming/hardening a larger diameter of column than the hole by inserting the grout through the injection pipe.