The transverse instability of the nonlinear Schroedinger equation, that is, the case that transverse perturbation is imposed on 1-dimensionally stable solution of nonlinear Schroedinger equation, was studied. It has been known that the characteristics of time evolution change at α=0. Since, I payed special attentions to the time evolution near α=0 and found that for α>0, the perturbation grows monotonously, while the perturbation grows oscillating for α<0. Two mechanisms which make the system unstable, were considered. From those mechanisms, a model composed of three coupled oscillators with different frequencies to one another, was suggested. And the model was found to exhibit the similar behavior to the transverse instability of nonlinear Schroedinger equation.
