A gas cylinder comprising: a base tube comprising a cavity; a spindle inserted into the cavity and performing a reciprocating motion in a longitudinal direction of the base tube; a cylinder mounted in the spindle and charged with gas; a piston dividing the cylinder into an upper chamber and a lower chamber; a valve portion sealing an upper portion of the cylinder and comprising a pipe holder having a cavity through which the gas is introduced; a flow prevention portion (140, 140a) inserted into the spindle and having an inner surface contacting a surface of the spindle; and a spindle guide disposed between the base tube and the spindle and mounted to comprise at least a portion of the flow prevention portion (140, 140a), wherein the spindle guide comprises a first mounting portion formed along an inner surface of the spindle guide in a circumferential direction, and a first stopper bump protruding from an inner side wall of the first mounting portion toward the spindle, wherein an inner circumferential surface of the flow prevention portion (140, 140a) adheres to the surface of the spindle, while an outer circumferential surface of the flow prevention portion (140, 140a) is not coupled to or contacts the surface of the spindle, and characterized in that an internal diameter of the first stopper bump is greater than a diameter of the spindle and is less than an internal diameter of the side wall of the first mounting portion, such that when the spindle performs an upward linear motion, the flow prevention portion moves upward along with the spindle and the first stopper bump comes in contact with an upper surface of the flow prevention portion such that the movement of the flow prevention portion is limited. a base tube comprising a cavity; a spindle inserted into the cavity and performing a reciprocating motion in a longitudinal direction of the base tube; a cylinder mounted in the spindle and charged with gas; a piston dividing the cylinder into an upper chamber and a lower chamber; a valve portion sealing an upper portion of the cylinder and comprising a pipe holder having a cavity through which the gas is introduced; a flow prevention portion (140, 140a) inserted into the spindle and having an inner surface contacting a surface of the spindle; and a spindle guide disposed between the base tube and the spindle and mounted to comprise at least a portion of the flow prevention portion (140, 140a), wherein the spindle guide comprises a first mounting portion formed along an inner surface of the spindle guide in a circumferential direction, and a first stopper bump protruding from an inner side wall of the first mounting portion toward the spindle, wherein an inner circumferential surface of the flow prevention portion (140, 140a) adheres to the surface of the spindle, while an outer circumferential surface of the flow prevention portion (140, 140a) is not coupled to or contacts the surface of the spindle, and characterized in that an internal diameter of the first stopper bump is greater than a diameter of the spindle and is less than an internal diameter of the side wall of the first mounting portion, such that when the spindle performs an upward linear motion, the flow prevention portion moves upward along with the spindle and the first stopper bump comes in contact with an upper surface of the flow prevention portion such that the movement of the flow prevention portion is limited.