Light source characteristic data representing the spectral characteristics and light distribution characteristics of a light source which illuminates a scene is input. Object characteristic data representing the spectral property of an object contained in the scene is input. Ray tracing is performed for respective rays having different wavelengths. A spectral radiance in each pixel of a computer graphics image representing the scene is calculated based on the light source characteristic data and the object characteristic data. The color signal values of each pixel are calculated from the spectral radiance in the each pixel, and the computer graphics image of the scene is generated.