Japan Aerospace Exploration Agency (JAXA) has developed the small demonstration solar sail spacecraft IKAROS (Interplanetary Kite-craft Accelerated by Radiation of the Sun), which will be launched in mid 2010. The main objective of this spacecraft is to deploy the 20m class sail membrane, and demonstrate the acceleration of a spacecraft by the solar radiation pressure (SRP) by means of that sail. It is important to model the SRP force adequately for the objective of navigation, especially for interplanetary spacecraft. In order to improve the model of the SRP torque induced by the sail membrane, the IKAROS project team plans to estimate the SRP torque parameters in orbit. In this paper, we present the approach to obtain the parameters needed for constructing the photon torque model through the analysis of the attitude dynamics.