PURPOSE: To prevent the leakage of a fluid when the connection is released by providing an inner pipe, a middle pipe and an outer pipe on an additional boring rod, making the inner pipe axially movable, and providing valve devices for opening and closing an inside conduit and a middle conduit.

CONSTITUTION: Each of additional boring rods 1... is provided with an inner pipe 7, a middle pipe 9 and an outer pipe 11 to form an inner conduit 6, a middle conduit 8 and an outer conduit 10. The boring rod 1 is further provided with a valve device for opening and closing each conduit 6, 8, 10, and the inner pipe 7 is made axially movable, so that the valve device is opened and closed according to the movement. All the valves are closed in the state where the boring rods 1... are separated, and opened according to the movement of the inner pipe 7 in the perfect connection of the boring rods 1... to allow each conduit 6, 8, 10 to communicate with each other.