This paper studies the nonlinear response of a parallel ferroresonant circuit. To carry out a transient analysis in parallel ferroresonant circuit, we apply Chua-type magnetization model to an inductance exhibiting saturation and hysteretic nonlinear properties of ferromagnetic materials, deriving a state variable equation and solutions by the backward Euler method with automatic modification. The characteristic values of the state transition matrix are calculated in each calculation step of Euler method in order to extract the chaotic characteristics. As a result, it is clarified that the chaotic behavior in the ferroresonant circuit is greatly concerned with the magnetic aftereffect of ferromagnetic materials.