PURPOSE: To aim at shortening the operating period by adhering a pit mouth block body to the inside of an existing tunnel, disposing a switching gate capable of being freely opened and closed on its front face and opening the switching gate immediately after the start of a shield machine so as to let the block body pass through therein.

CONSTITUTION: A start gate device 1 is provided in the start position in an existing tunnel 2, and a shield machine 3 is disposed facing the start gate 1 in the state of a switching gate closed. The shield machine 3 is started by the reaction from the existing tunnel 2 on the facing surface side of the start gate device 1, and the switching gate 5 is gradually opened until just before being touched by the facing of the shield machine 3. When the shield machine 3 passes through the gate device 1, a pit mouth sealant 45 is stopped by the contact with the peripheral surface of the machine 3. As a result, the shield machine 3 can be started while aiming at dead water so as to be used even at the bottom of the sea.