We propose a novel method to simulate image noise caused by gamma-ray irradiation. Monte Carlo simulation is utilized to calculate the interaction between the gamma-ray and the image sensor, and the energy deposit in each pixel is estimated. A converter module is proposed for generating image noise from the energy deposit. The conversion is designed so that the converted energy deposit has a similar image histogram to the real gamma-ray image noise. The real gamma-ray image noise is obtained by using circular fisheye cameras in gamma-ray irradiation tests. We demonstrate the effectiveness of the proposed method in experiments. We believe that this study would be beneficial for the development of methods for decommissioning the Fukushima Daiichi Nuclear Power Plant by the research community. Our source code is available at https://github.com/matsuren/pixel_noise_sim_geant4.