By measuring the gastrointestinal motility, this study is aimed at obtaining a mathematical model of electrogastrography (EGG) and speculating factors to describe the diseases resulting from constipation and erosive gastritis. Initially, we applied the Double-Wayland algorithm to the EGG in order to measure the degree of determinism. As a result, we could not decide whether a chaotic process is appropriate for the mathematical model of the EGG. On the other hand, the waveform of the electric potential in the Cajal cells is similar to the graphs of numerical solutions to the van der Pol equation (VPE). Hence, we added the VPE to a periodic function and random white noises named after the intestinal motility and other biosignals. We rewrote the stochastic differential equations (SDEs) into difference equations, and the numerical solutions to the SDEs were obtained by Runge-Kutta-Gill formula as the numerical calculus, where we set the time step and initial values to be 0.05 and (0, 0.5), respectively. Pseudo-random numbers were substituted in the white noise terms. In this study, the pseudo random numbers were generated by the Mersenne Twister method. These numerical calculations were divided into 12000 time steps. The numerical solutions and EGG were extracted after every 20 steps. The EGG and numerical solutions were compared and evaluated by the Lyapnov exponent and translation error. The EGG was well described by the stochastic resonance in the SDE. [J Physiol Sci. 2008;58 Suppl:S141]