A vibration suppressing method and a vibration suppressing device are disclosed. After a tool is attached to a main spindle, a modal parameter of the tool or a workpiece is computed. Thereafter, a relation between chatter frequency and phase difference is calculated as an approximation formula based on the obtained modal parameter and machining conditions. If chatter vibration occurs after initiation of the machining, a chatter frequency corresponding to a target phase difference is obtained using the approximation formula, and based on the obtained chatter frequency, the number of tool flutes and the main spindle rotation speed, the optimum rotation speed is calculated. The rotation speed of the main spindle is then changed in accordance with the obtained optimum rotation speed.