The precise energy-dependent response curve of the imaging plate to low-energy photons was obtained by reciprocally combining a calculation and an experiment. The calculation was carried out using the Monte Carlo particle transportation code EGS5. The experiment was carried out using monochromatic photons at the photon factory in KEK. Initially, there was a discrepancy between the calculation and the experiment because of light attenuation in the phosphor layer in the read-out process. However, by setting the attenuation coefficient of this light attenuation to 0.013 μm-1 and decreasing the imaging plate (IP) response exponentially with depth from the phosphor surface, it was indicated that the calculation result basically follows the experimental result. By using the response curve obtained in this paper, radiation dose can be evaluated accurately in the dose estimation using IP.