PURPOSE: To aim at shortening the constructing period by drilling ahead a shield machine, with the front cover ring located in the receding state and the tip formed cover ring matching the side surface of an existing tunnel expecting docking in the protruding state so as to shield the entire peripheral surface for connection.

CONSTITUTION: A tunnel is constructed toward an existing tunnel 2 having a shield machine 1 positioned at the head and is dug ahead having a cover ring 12 enclosed in a shield machine 1 and an over cutter 16 in the state of protrusion. When the cutter part 14 of the shield machine 1 approaches the existing tunnel 2, the over cutter 16 is enclosed in the cutter part 14. In addition, when the cover ring 12 is penetrated into the ground in the state of protruding toward the peripheral surface of the existing tunnel 2, the entire front surface 13 is adhered to the outer peripheral surface of the existing tunnel 2 so as to shield the entire surface. As a result, a connecting tunnel 4 is connected to the side of the tunnel 2, and this can be operated even at the bottom of the sea.