A delay in installing rock bolts means generating loosen area around tunnels. There are, however, still uncertainties about the effect of rock bolts in loosen area A example is whether we should install the longer bolts than the loosen area or not. In this paper, to obtain the influence of time for installing rock bolts on the effect, trapdoor tests with various bolt length and time for installing bolts and numerical analyses using 2D-FDM were carried out. It was found that in deep tunnel the longer bolts than the loosen area have almost the same effect irrespective of the time. On the other hand, in shallow tunnel, the time has an influence on the bolts effect.