This paper describes the effect of the thickness of the recording medium on magnetic force acting on the magnetic toner in magnetography with longitudinal recording. The thickness of the magnetic recording medium is one of the important factors in forming image. It has been reported that increasing the thickness leads to increasing the magnetic force acting on the magnetic toner from the isolated transition region in the recording medium. However, in the case of high resolution, the effect of the demagnetization phenomenon increases due to the closer adjacent transition regions. In the demagnetization phenomenon, increasing the thickness acts to decrease magnetization of the recording medium, which in turn acts to make the magnetic force lower. In this study the optimum thickness is found, considering the above conditions. While increasing the thickness acts to increase the magnetic force in the thinner thickness than the optimum thickness, increasing the thickness acts to decrease the magnetic force in the thicker thickness than the optimum thickness.