Variable preload technology involving varying preloads with respect to the spindle speeds and machining conditions is the most suitable preload method for spindles that require extensive rotation. The necessity of variable preload technology will continue to grow as the spindle speed and efficiency increase in order to improve the productivities of machine tools. This study attempts to develop a variable preload spindle that varies the preload with respect to the rotation number, allowing high speed rotation, and improves the stiffness characteristics. The authors designed and fabricated a variable preload device composed of an electromagnetic actuator and other components. We then evaluated the performance of the fabricated variable preload device with an experiment using load cells and subsequently determined the performance of the device and identified areas requiring improvement through a performance evaluation. Finally, we developed a variable preload spindle by applying the produced variable preload device. Smooth operation was verified through performance evaluations of the variable preload spindle, and stiffness improvement was verified through preload variation.