This paper presents a new concept of the seismic countermeasure for a structurally special part of tunnel such as ramp of road tunnel and a crossover of double-truck railway tunnel, in which a flat and large cross-sectional tunnel and a junction of tunnels appear. Since  seismically high stability is required for it, a series of centrifuge model tests of cross-sectional tunnel and ground system with following countermeasures were conducted in the transverse direction: (1) isolation layer around outer surface of tunnel; (2) ground improvement by cement-solidification surrounding tunnel; and (3) combination of the former ones. As a result, combination method can be assumed to be most effective, and the effectiveness of the seismic deformation method in the seismic design of such tunnels is experimentally verified. Additionally, a cost-effective material for the seismic isolation layer is proposed with considering its cyclic properties, which can be applied to a flat cross-sectional tunnel and a junction of tunnels.