To meet the demands of high-efficiency and high-precision machining, machining centers with high-speed spindles are used for machining die. The more the spindle speed increases, the more the chatter-vibration-resistance of the spindle-system composed of a spindle and a tool holder decreases. It is mainly attributed to the centrifugal force acting on the spindle system. At higher spindle speeds, the spindle speed makes the contact state between the tool holder taper shank surface and the spindle taper surface worse, resulting in a decrease in grasping force of the tool holder, which is also not preferable in view of workers' safety. This study aimed to improve the chatter-vibration-resistance of BT-type tool holders with improving the contact state between the taper surfaces at higher spindle speeds. A method to improve the bending rigidity of the tool holder was proposed by numerical examinations of the contact state.