To provide a joint sheath tube for preventing the leakage of joint mortal that is filled between a precast concrete beam and a pole, which has such a structure as not malfunctions when not in use.

A pole is jointed to a beam using a joint sheath tube which comprises a double cylinder type outside tube formed on the outer periphery of the sheath tube with a bottomed prescribed space, and comprises a double cylinder type inside cylinder which is inserted and fitted in the space of the outside cylinder formed by the outer cylinder and the sheath tube and forms the bottomed prescribed space communicating with a space which is formed by the outside cylinder. A breakable sheet is pasted on the abutting surface of the inside cylinder that abuts on the pole. After the beam is spanned, a swelling agent is injected in the space formed by the inside cylinder, so that the swelling agent flows into the space formed by the outside cylinder, to extrude the inside cylinder toward the pole for abutting, and thus the joint mortal is installed between the pole and the beam.