Exact solution for equation of motion is obtained for two-level electronic system with Gaussian frequency modulation under laser irradiation of any intensity. This solution is valid for the fast (weak) and the slow (strong) modulation and for any intensity of laser field including both the weak and strong limits. In terms of this solution, we can calculate the nonlinear optical response of two-level system both in the frequency and the time domains. In this communication, we show the change of absorption spectrum against laser intensity for any degree of the frequency modulation including both the fast and slow limits.