The cost of NATM can be decreased by the selection of suitable support system depends on geological condition. In this paper, the way how we can select a suitable support system is proposed based on convergence measurement, which is very sensitive to the behaviour of rock mass. The authors find out the relationship between allowable limit of convergence derived from critical strain and allowable limit of support stress derived from FEM analysis. Using this relationship we can choose ideal support system. The validity of this method is confirmed at Nou tunnel.