PURPOSE: To make it possible to hold uncured concrete directly after it is placed and promptly assemble reinforcement in a succeeding process and prevent the concrete placed outside a cylinder-shaped inner side form from leaking out to a weir structure side.

CONSTITUTION: Cylinder-shaped inner side forms are formed one after another near a working face of a tunnel to be excavated with an excavator 1 and lining concrete is consecutively placed outside an inner side form 4 in the longitudinal direction of the tunnel by means of a sliding concrete placing pipe 28. There is laid out a weir structural body which provides an outer peripheral grip ring 13 and a middle grip ring 12 and an inner peripheral grip ring 11 inside a skin plate 2 of the excavation in such a fashion that this placed concrete may be pushed backward. There is laid out a ring-shaped rubber seal material, which holds this weir structural body with a gripper holding device 7 against the push force produced by the placed concrete and provides a slit which allows reinforcing bars to pass through, but does not allow the concrete to pass through.