PURPOSE: To enhance workability by closing the front side of concrete with a rubber tube and supporting the concrete with a presser hardware and holding a concrete casting form side of the rubber tube with an adjustable metal fitting.

CONSTITUTION: An inner form B is installed between primary lining segments C by way of a specified concrete placing space. Then, after a rubber tube 1 is installed, an angle-shaped presser metal fitting 2 and a circular-shaped adjustable metal fitting 3 are mounted to an end face on the gable side of the form B. Then, air or the like is filled up in the tube 1 so as to expand the tube and pressure weld the tube with a bottom plate 32 of the metal fitting 3. Concrete is placed between the form B and primary lining C, thereby constructing secondary concrete A. During that time, the metal fitting 3 is moved in the direction of concrete placement under control, thereby closing the gable side of the secondary lining A without any clearance. It is, therefore, possible to close the gable side in conformity with the thickness of concrete placing.