One of the more useful tools for better understanding population dynamics is the phase response curve (PRC). Recent physiological experiments on the PRCs using real neurons showed that different shapes of the PRCs are generated depending on the perturbation, which has a finite amplitude. In order to clarify the origin of the nonlinear response of the PRCs, we analytically derived the PRCs from single neurons by using a spike response model. We clarified the relation between the subthreshold membrane response property and the PRC. Furthermore, we performed numerical simulations using the Hodgkin-Huxley model and their results have shown that a nonlinear change of the PRCs is generated. Our theory and numerical results imply that the nonlinear change of PRCs is due to the nonlinear element in spike time shift of firing neurons induced by the finite amplitude of the perturbation stimuli.