For an application to a dose-distribution measurement, the response of an imaging plate (IP) has been studied with proton beams which are routinely utilized for the radiation therapy. The upper limit of measurable proton dose by an IP system is primarily controlled by the readout range of the scanner that is used. Within this limit, reasonable linear response of an IP to proton dose to water is maintained. Fading curves are neither sensitive to a small change of room temperature (22-26°C) nor to a variation of proton dose (0.0108-0.132 Gy). Reproducibility of the PSL intensity is fairly good if both the fading characteristics and the lot-dependence of the sensitivity of each IP are taken into account carefully. Stopping power dependence of the IP response has been examined at different positions in a Bragg curve.