A simplified estimation method for first excursion probability of mechanical systems subjected to earthquake excitations is proposed. First, by a simulation method, first excursion probability of a single-degree-of-freedom system is estimated using stationary random noise as input. When the tolerance level is normalized by the expected value of the maximum response, which is well known as the average response spectrum, the variation of first excursion probability for various values of damping ratio and natural period is very small. These characteristics can be seen for the system subjected to nonstationary random noise. Second, the above mentioned result is examined by a theoretical method. Finally, the obtained result is applied to a secondary system. In this case, the variation of first excursion probability for various values of damping ratio and natural period is very small when the tolerance level is normalized by the expected Value of the maximum response of the secondary system.