A femtosecond laser machining has been utilized to fabricate various optical devices inside transparent material. To elucidate the mechanism of the femtosecond laser induced structural change inside a glass, we have observed the dynamics of the structural change and detected pressure wave propagation by a Transient Lens (TrL) method. Although the pressure wave generation has been already observed in our previous study, the analysis to obtain the temporal evolution of the refractive index distribution change was rather complex. In this study, we show alternative analysis method for TrL signals of bulk modification inside glass by femtosecond laser. From the analysis, the dynamics of the structural change including refractive index change at the center and shape of pressure wave becomes clear.