TENNNOZAN TUNNEL were constructed as two lane highway tunnels 35years ago. An extension of cross section was planned because of modern traffic increase. The initial construction had experienced large deformation and high pressure on concrete lining due to landslides in the west portal. On the actual extension, the collapse of sand layer and large deformation of old tunnel lining occurred. It was clarified that this large deformation is due to loosening of sand layers by excavation and successive stress redistribution in fault zone. Finally, the Authors adopted AGF method as a countermeasure and completed the extension of cross section.