In this study, we developed an active-caster mechanism using a differential mechanism with a twisted-timing-belt and verified its operation with a prototype. The active caster enables omni-directional movement using standard tires, effectively addressing these concerns. However, conventional active casters drive the wheels and steering separately with different motors, resulting in inefficiencies in motor operation. To overcome this, we developed a mechanism that integrates the power of two motors using a differential mechanism with spur gears, distributing power to both the wheels and steering, and applied this technology to devices such as wheelchairs. Nevertheless, this approach faced reduced transmission efficiency due to the increased number of gear stages. Therefore, in this study, we propose a novel active caster mechanism employing a differential mechanism with a twisted-timing-belt. We elucidated the operational principles of this mechanism and derived a kinematic model. Furthermore, we constructed a prototype and validated its movement on a flat surface, confirming the feasibility of achieving omni-directional movement with our proposed mechanism.