The purpose of this research is to establish a concept of the factor of safety of tunnels which is a quantitative estimate of the stability of tunnels. Based on this concept, a numerical technique which calculates the factor of safety of tunnels was developed. To obtain the safety factor of a tunnel, the strength reduction technique in which a series of analyses are repeated with reduced ground strength until the tunnel collapses were employed. With this technique, the failure plane, as well as the factor of safety, can be obtained without prescribing the trial failure plane. Analyses were conducted with FLA $C^{2D}$(ver3.3), a geotechnical analysis program which is based on the finite difference method. From the result, the location of plastic zones, the maximum convergence and the maximum stress generated in the support system were also analyzed. The result shows that factors of safety are higher for the 1st and 2nd rock classes, and lower for the lower rock classes. Furthermore, factor of safety is higher when $K_{0}$ =0.5 compared to at in case of $K_{0}$ =2.0. Through this research, it is found that the factor of safety defined in this research can be used as a good quantitative index representing the stability of tunnels. Also, close examination of the results can help adjustment of the quantity and location of additional supports.s.