A vent pipe isolation device (1; 101) for temporarily isolating a vent pipe of a liquefied gas storage tank, the vent pipe isolation device comprising: a storage pipe (2; 52; 102); an operation pipe (54a, 54b); an intermediate support portion (3; 103); a balloon attachment portion; and a balloon, wherein the storage pipe (2; 52; 102) has a closing flange on an opening side thereof, and stores the intermediate support portion (3; 103) and the balloon attachment portion, the intermediate support portion (3; 103) and the balloon attachment portion are arranged in the storage pipe (2; 52; 102) along an identical central axis, the intermediate support portion (3; 103) and the balloon attachment portion are movable in the storage pipe (2; 52; 102) in an axial direction, and the intermediate support portion (3; 103) has a front end side opening downward, the intermediate support portion (3; 103) is fixed to the operation pipe (54a, 54b) at an intermediate position thereof and on a closed portion side, the balloon attachment portion has a first balloon attachment member and a second balloon attachment member at both ends thereof and a hinge portion at an intermediate position, a side forwardly of the hinge portion being bendable, the operation pipe (54a, 54b) is bendable at a fixing portion on a side of the intermediate support portion (3; 103) that is adjacent to the balloon attachment portion, the first balloon attachment member and the second balloon attachment member are fixing members for airtightly fixing the balloon, at least one of the first balloon attachment member and the second balloon attachment member being movable in the axial direction, the operation pipe (54a, 54b) includes therein a gas pipe for supplying an inert gas to the balloon from an outside and discharging the inert gas from the balloon to the outside, the balloon includes an inner membrane, a reinforcing substrate, and an outer membrane, the balloon having a cylindrical shape or a truncated cone shape having openings at both ends thereof, and being inflated when an inert gas is injected into the balloon with the openings sealed, a material of the inner membrane and the outer membrane is cold resistant rubber, the reinforcing substrate is formed of a fiber bundle and has a network structure, and the reinforcing substrate is sandwiched between the inner membrane and the outer membrane. a storage pipe (2; 52; 102); an operation pipe (54a, 54b); an intermediate support portion (3; 103); a balloon attachment portion; and a balloon, wherein the storage pipe (2; 52; 102) has a closing flange on an opening side thereof, and stores the intermediate support portion (3; 103) and the balloon attachment portion, the intermediate support portion (3; 103) and the balloon attachment portion are arranged in the storage pipe (2; 52; 102) along an identical central axis, the intermediate support portion (3; 103) and the balloon attachment portion are movable in the storage pipe (2; 52; 102) in an axial direction, and the intermediate support portion (3; 103) has a front end side opening downward, the intermediate support portion (3; 103) is fixed to the operation pipe (54a, 54b) at an intermediate position thereof and on a closed portion side, the balloon attachment portion has a first balloon attachment member and a second balloon attachment member at both ends thereof and a hinge portion at an intermediate position, a side forwardly of the hinge portion being bendable, the operation pipe (54a, 54b) is bendable at a fixing portion on a side of the intermediate support portion (3; 103) that is adjacent to the balloon attachment portion, the first balloon attachment member and the second balloon attachment member are fixing members for airtightly fixing the balloon, at least one of the first balloon attachment member and the second balloon attachment member being movable in the axial direction, the operation pipe (54a, 54b) includes therein a gas pipe for supplying an inert gas to the balloon from an outside and discharging the inert gas from the balloon to the outside, the balloon includes an inner membrane, a reinforcing substrate, and an outer membrane, the balloon having a cylindrical shape or a truncated cone shape having openings at both ends thereof, and being inflated when an inert gas is injected into the balloon with the openings sealed, a material of the inner membrane and the outer membrane is cold resistant rubber, the reinforcing substrate is formed of a fiber bundle and has a network structure, and the reinforcing substrate is sandwiched between the inner membrane and the outer membrane.