Imasato-Daiichi Tunnel is the twin parallel tunnels with a large cross section under small overburden which are located extremely close to existing structures. Concerning the construction of the tunnels, a lot of technical difficulties were anticipated due to buildings right above the tunnels. Thus, the design of high-strength support system and improvement of pillars between the tunnels with preceding grouting operations were made to assure mechanical stability. These innovations contributed to the successful construction of large twin tunnels very close to existing structures, which led to the establishment of design and construction methods for this kind of tunnels.