A device for blocking a flow passage without interrupting flow, the device comprising: a fluid pipe having a branch port formed therein; a branch pipe portion that is in communication with the branch port and has a valve member; a sealing case that is to be in communication with the branch port and that is to be detachably connected to the branch pipe portion; an insertion guide tube that is inserted into the sealing case; a shaft that is inserted into the sealing case and the insertion guide tube so as to be slidable along an axis of the branch pipe portion while a sealed state is maintained and that includes a through flow passage inside; a blockage bag that is fixed to a leading end portion of the shaft, the leading end portion being located inside the insertion guide tube, and that is deformed and elastically expanded so as to increase in diameter by a diameter-increasing fluid that is supplied via the through flow passage to close an internal flow passage of the fluid pipe into which the blockage bag is inserted; a push operation mechanism that pushes and moves the shaft against a fluid pressure inside the fluid pipe; and an axis adjustment portion that is fixed to the leading end portion of the shaft or a base end region of the blockage bag, the leading end portion and the base end region being located inside the insertion guide tube, and that can make sliding contact with an inner surface of the insertion guide tube, wherein the insertion guide tube, with the blockage bag in a reduced-size state being housed in the insertion guide tube, can be moved inside the sealing case and inside the branch pipe portion along the axis of the branch pipe portion by the push operation mechanism, until the insertion guide tube is brought into an abutting position in which at least a part of a leading end portion of the insertion guide tube abuts against an outer surface-side peripheral edge portion of the branch port on an outer circumferential surface (1 a) of the fluid pipe, and the blockage bag that is housed in the insertion guide tube in the abutting position can be inserted into the internal flow passage of the fluid pipe from the inside of the insertion guide tube via the branch port along the axis of the branch pipe portion by the push operation mechanism, and in a state in which the blockage bag is located at a predetermined internal flow passage blocking position, the axis adjustment portion is located in a portion corresponding to the branch port in a radial direction of the fluid pipe. a fluid pipe having a branch port formed therein; a branch pipe portion that is in communication with the branch port and has a valve member; a sealing case that is to be in communication with the branch port and that is to be detachably connected to the branch pipe portion; an insertion guide tube that is inserted into the sealing case; a shaft that is inserted into the sealing case and the insertion guide tube so as to be slidable along an axis of the branch pipe portion while a sealed state is maintained and that includes a through flow passage inside; a blockage bag that is fixed to a leading end portion of the shaft, the leading end portion being located inside the insertion guide tube, and that is deformed and elastically expanded so as to increase in diameter by a diameter-increasing fluid that is supplied via the through flow passage to close an internal flow passage of the fluid pipe into which the blockage bag is inserted; a push operation mechanism that pushes and moves the shaft against a fluid pressure inside the fluid pipe; and an axis adjustment portion that is fixed to the leading end portion of the shaft or a base end region of the blockage bag, the leading end portion and the base end region being located inside the insertion guide tube, and that can make sliding contact with an inner surface of the insertion guide tube, wherein the insertion guide tube, with the blockage bag in a reduced-size state being housed in the insertion guide tube, can be moved inside the sealing case and inside the branch pipe portion along the axis of the branch pipe portion by the push operation mechanism, until the insertion guide tube is brought into an abutting position in which at least a part of a leading end portion of the insertion guide tube abuts against an outer surface-side peripheral edge portion of the branch port on an outer circumferential surface (1 a) of the fluid pipe, and the blockage bag that is housed in the insertion guide tube in the abutting position can be inserted into the internal flow passage of the fluid pipe from the inside of the insertion guide tube via the branch port along the axis of the branch pipe portion by the push operation mechanism, and in a state in which the blockage bag is located at a predetermined internal flow passage blocking position, the axis adjustment portion is located in a portion corresponding to the branch port in a radial direction of the fluid pipe.