Mobile satellite communication systems are attracting much attention as an effective way to provide broadband communications in planes, ships, and long-haul trains in which economical services are not effectively provided by terrestrial systems. Such a system usually requires a wide frequency bandwidth and a satellite with high electrical output power, but these resources are severely restricted. A multi-beam system with frequency reuse is able to spread the usable bandwidth virtually. Applying an adaptive modulation method to the system achieves efficient use of resources. In this system, optimally assigning resources to beams is indispensable in enhancing the system capacity. However, this optimization method becomes complex since it is necessary to consider the influence of inter-beam interference which may depend on the variation in the communication traffic distribution. We propose a new adaptive resource assignment method in order to overcome this complexity and to enhance the system capacity. In the proposed method, a bandwidth vector and an approximate curve of the spectrum efficiency are introduced. Based on case studies assuming 19 beams, we show that the proposed method achieves 4.2 times higher system capacity than that of a system in which resources of each beam are fixed.